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Old 27th Nov 2012, 11:45 pm   #21
Chris Wilson
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Default Re: EIP 545A 18GHz counter questions. Signature analysers?

OK, trhanks for that Jeremy, I have had a good look at board A108 the chip is missing from, and apart from the mysterious pair of multi turn pots the schematic parts list seems to say are "not used" I can see no anomalies. I'll post the results of fitting this device.

Anyone any ideas what might cause the GPIB board to kill the display? It has two turn wheels and was set to 03, but the manual says the default is 19. I tried 19, same result, a display of all dashes. The three tantalum caps on it check out ok, there are only 3 ceramic caps and eight same value resistors, the other components are all socketed chips.
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Old 29th Nov 2012, 10:21 am   #22
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Default Re: EIP 545A 18GHz counter questions. Signature analysers?

The replacement for the missing U6 chip arrived this morning and it now completes the 200 MHz self test It will read up to the limit of my frequency generator
on Band 3 which is only 1040 MHz, but looks promising.

Band 2 also works, but seemingly has an anomaly. I cannot get Band 2 to read below
185.000 MHz. If I input 184.900 it won't read it and displays zeros.
185 and up is OK... Weird. It should read from 10 MHz up to 1 GHz. It'll go to 1 GHz plus AOK.

Band 1 appears to work fine.

These tests are with both my GPS referenced external standard and the internal reference used.

So Band 2 appears to have a further issue. Will refer to the
manual and see if I can fathom it out. Any ideas as to why Band 2 will only
read from 185 MHz upwards please? Thanks.
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Old 30th Nov 2012, 12:12 am   #23
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Default Re: EIP 545A 18GHz counter questions. Signature analysers?

I have isolated the issue to the A109 board, where the Band 2 input is connected to the board. Until the unit warms up Band 2 is deaf and needs plenty of millivolts input to trigger. Once warm it settles down to the makers spec on how much input at what frequencies it should need to trigger. I used freeze spray (which has run out, of course...) to isolate a small area, and the fine tip of a de soldering iron connected backwards to the pump, so it blows a fine jet of hot air. The problem is in the area marked on the schematic attached and at http://www.gatesgarth.com/heat.jpg. As I have no riser board access is terribly limited. Has anyone with more knowledge than myself got a hunch what components in this region might respond to heat and cold and make the band a bit deaf until it warms up? I'm guessing it could pretty much be any of them Hopefully it's not that Schotkky diode (spelling?) as I can't seem to cross reference it to anything available. Thanks
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Old 30th Nov 2012, 12:51 am   #24
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Default Re: EIP 545A 18GHz counter questions. Signature analysers?

I can offer a guess at some system info here if it helps?

The core 200MHz counter on its own obviously can't respond to frequencies above this limit so the system has to resort to using the mixing process to see higher frequencies like 1GHz etc.

From a system point of view this means the system has to fish/hunt for the 1GHz signal by tuning the VCO across its range to try and get a downmix/match that is in the right range for the 200MHz counter to zoom in on. So one requirement to kick off the 'hunt' is that the counter needs to know if a signal is present at port 2 in the first place. Otherwise the system processor would be busy fishing all the time for a signal that isn't there....

So my interpretation of the circuit is that it has a broadband diode detector at the port 2 input (J4 connector) that feeds to a comparator U1 that then triggers a flag/interrupt back at the big 40 pin PIA chip (MC68B21P)

The processor presumably polls for this flag or responds to the PIA driven interrupt to kick off the fishing routine.

So you need to ensure the signal you apply here at port 2 is big enough to trip the diode detector/comparator to make it go into 'fish mode' and start tuning the VCO to find the correct PLL frequency.

A good place to look would be at the comparator U1 pin 6 as this should flip its voltage level by quite a few volts when a signal of large enough amplitude is detected by the detector diode at the input port J4.

Apologies in advance if you already realise all this...

Last edited by G0HZU_JMR; 30th Nov 2012 at 12:57 am.
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Old 30th Nov 2012, 1:11 am   #25
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Default Re: EIP 545A 18GHz counter questions. Signature analysers?

I would also be tempted to measure the voltages around the BFR90.

This is a slightly curious circuit because it is based on a common base amplifier with a very low collector impedance formed by the 51R resistor and the mixer input impedance.

This BFR90 is presumably some form of rugged active attenuator/limiter ahead of the mixer but maybe it has been overloaded and damaged in the past?

Can you measure the voltages at the base, emitter and collector with no signal injected?

This circuit should behave very predictably at DC and RF so if there is a problem with the transistor or the resistors around it then it will hopefully show up in the basic DC bias checks.

eg
base = 0V (i.e. maybe -5mV?)

emitter = -0.8V

Collector = 3.75V

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Old 30th Nov 2012, 1:28 am   #26
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Default Re: EIP 545A 18GHz counter questions. Signature analysers?

Thanks Jeremy, will see if I can get test clips on and get the board back down in its socket. I have changed C2 and C3 with no change, so I'll have a go at the voltages. I am out tomorrow night so it will be Saturday night before I can get at it again. At least U6 has got the basics running now. Apparently one of the selling points of this counter was its ruggedness and resistance to abuse from too high an input level. Maybe that explains it? I am pretty sure the temperature related issue is in this localised region. Thanks again for all your time.
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Old 30th Nov 2012, 1:38 am   #27
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Default Re: EIP 545A 18GHz counter questions. Signature analysers?

That's OK...
I'd also check that the 150R resistor R1 is still 150 ohms because it looks to be quite vulnerable at 0.125W rating. Maybe it has changed value to a high resistance and drifts with temperature?

The BFR90 circuit Q1 is quite interesting and maybe it proves more rugged than an equivalent resistor based attenuator. I've probably got a few TFM-2 mixers kicking about assuming the mixer MX1 is a regular minicircuits TFM-2 mixer.
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Old 30th Nov 2012, 2:05 pm   #28
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Default Re: EIP 545A 18GHz counter questions. Signature analysers?

Checked R1 and it's in spec and stable when warmed. I found it all but impossible to probe the BFR90, short of soldering fly leads on to bring test points up above the board, *BUT* I finally tracked down what is changing the sensitivity. Please see attached or at http://www.gatesgarth.com/heat2.jpg

It's a 2N4126 component number Q6 If I just touch it with a piece of stiff, heavy copper wire wound round the tip of a small 15W iron, it changes state almost immediately and the Band 2 is very sensitive again. As soon as it cools it reverts to the insensitive state. But bear in mind above 185 MHz it seems pretty sensitive hot or cold...If the counter is left to warm up naturally and i cool this device with some spray, it goes insensitive again.

I am not 100% sure what it's function is, but it's definitely iffy. I have removed it, and isolated on my Peak semiconductor tester it varies gain form 170 cold to 190 warm, and it will suddenly go to a gain of just 4 if it gets a bit warmer still.

R10 is also playing up. It should be a 43K 2% but measures 32K cold and warmed a touch changes to 4.8K ! It should have a 1 PPM/C temperature coefficient, too, so something has happened to that as well.

The mixer you spoke of Jeremy is marked MCL TFM2 so I guess it IS a Mini Circuits one. It doesn't seem temperature affected at all, though.


I am not sure if any previous abuse would have damaged diode CR1, shown as a ND4991 ?

Unsure whether to buy the used board from China or persevere with this one.

Thanks for your help, it's very interesting to me, I would never have dreamt of chasing an issue like this down last year.
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Old 30th Nov 2012, 3:47 pm   #29
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Default Re: EIP 545A 18GHz counter questions. Signature analysers?

Hi Chris
I think you threw me a bit when you pointed to the area around the BFR90 and diode as being the problem area as this didn't really stack up with your symptoms but you seemed sure it was a fault here.

But it looks like the freezer was maybe tripping Q6 or something else instead.

Either way, the best way to chase this fault is to understand how the counter's frequency plan works across all of range 2. Here's how I understand it. (I could be wrong though...)

LOW RANGE OF RANGE 2
For frequencies below about 185MHz? range 2 does NOT need a mixer at MX1. It can cope with the frequency direct because the core counter can work to 200MHz ballpark.

So for low range 2 frequencies, MX1 is presumably DC biased (by Q12?) to stop it behaving as a mixer and to make it into a low loss path 'straight through' i.e. like a switch. So the RF signal bypasses through it with no frequency conversion and then it can feed into the core counter.

MID RANGE OF RANGE 2
Here it needs to use the VCO as a phase locked local oscillator and MX1 acts as the mixer because the counter can't see frequencies this high without a downconversion in the mixer MX1.

However the VCO signal has to get through 'doubler' MX2 before it can be connected as the LO for MX1. It needs to get through WITHOUT being doubled.

So I think the processor does the same DC bias trick to MX2 in that it DC biases it as a straight through switch rather than act as a doubler. So it becomes kind of transparent to RF. Q6 does this DC bias trick to MX2 on this range. Q7 acts as some kind of belt and braces VCO signal spoiler here as well by switching/crowbarring RF amp Q5.


HIGH RANGE OF RANGE 2 (up to 1GHz)
Here the VCO runs out of range because the HIGH range of range 2 (1GHz?) is too high for the raw VCO. i.e. the VCO can't tune high enough in frequency on its own.

So the A109 board doubles the VCO frequency using MX2 so suddenly you get the required range to make range 2 work up to 1GHz. It doubles it by feeding the VCO to both ports 1 and 2 of TFM-2 MX2 to produce a sum of double the frequency.

Here's some initial diagnostic checks:

Measure the DC voltages at TP2, TP3 and TP4 and TP11 as you drive the counter with the sig gen at frequencies within range 2.

They should change status because these are the PIA control voltages to make all of the above 'happen' correctly assuming the circuits are healthy. So you need to establish if there is a control issue or if something is indeed wrong with a transistor like Q6.

However, it does sound like Q6 is duff and also R10 obviously has an issue. I assume you are testing it for resistance when out of circuit?
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Old 30th Nov 2012, 7:24 pm   #30
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Default Re: EIP 545A 18GHz counter questions. Signature analysers?

When you are trying to measure range 2 sensitivity with a low frequency going in to the counter range 2 input? eg 50MHz, have you measured the DC voltage at each side of R57 (in the collector of Q12)

You should see a ballpark 10V dropped across R57 and this would indicate about 5mA of DC bias current being fed into the mixer IF port (also used as the LO port in other range 2 modes)

This bias should allow MX1 to pass the 50MHz through the mixer directly.
Can you probe each side of the mixer to prove the 50Mhz signal is getting through the mixer OK? i.e. with a 100MHz scope?

Then have a look at the voltages at R27 to see what Q6 is doing in terms of bias injection to MX2. Also repeat the R27 test when you do your hot/cold test of Q6.

Or you could just swap Q6 and see if that fixes the odd behaviour here?
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Old 1st Dec 2012, 1:21 pm   #31
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Default Re: EIP 545A 18GHz counter questions. Signature analysers?

Thanks for all the great help again Jeremy. I have Q6 off the board so rather than refit it and keep heating the board I am awaiting a new one on Monday / Tuesday. I don't have anything that will cross reference satisfactorily, as far as I can tell. I'll do the measurements you suggest when the device is fitted, and try and learn something about how this little lot is supposed to work. I was confused about the area where the heat got things going, given the boards are hard against one another, or against alloy screening panels, it's very hard to direct heat accurately and Q6 had a low latency so it changed even when heat was elsewhere in the vicinity. I have some new resistors to replace R10 from, so will do that as well, as it certainly changes value quite wildly if you warm it slightly.

I was testing R10 with a leg off the board, yes.

Ill do the other tests in your earlier reply as well, even if it appears to work OK, again to try and get a better grasp of how it works. Can't thank you enough, brilliant guidelines, cheers Jeremy!
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Old 4th Dec 2012, 1:46 pm   #32
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Default Re: EIP 545A 18GHz counter questions. Signature analysers?

New Q6 and new R10 fitted, problem just the same. Warming the new Q6 brings the display below 500 MHz back.

Voltage tests as you suggested done, results below all with a 30mV emf input to the Band 2 socket:

TP2 No input signal 3.4 / 3.5V Displays zeros
TP2 10 MHz 3.4 / 3.5V Displays zeros
TP2 50 MHz 3.4 / 3.5 V Displays zeros
TP2 100 MHz 3.4 / 3.5V Displays zeros
TP2 400 MHz 3.4 / 3.5V Displays zeros
TP2 500 MHz 0.06V Displays frequency
TP2 900 MHz 4.9 / 5.0V Displays frequency

For TP2 the change over seems circa 450MHz

*** BUT *** Input 650 MHZ and TP2 shows 4.2 / 4.3V and displays zeros Warming Q6 makes no difference

TP 3 No input signal 1.5V
TP3 10 MHz 1.5V Displays zeros
TP3 50 MHz 1.5V Displays zeros
TP3 100 MHz 1.5V Displays zeros
TP3 400 MHz 1.5V Displays zeros
TP3 500 MHz 4.1V Displays frequency
TP3 900 MHz 0.0V Displays frequency

TP4 No input signal 4.5V Displays zeros
TP4 10 MHz 4.5V Displays zeros
TP4 50 MHz 4.5V Displays zeros
TP4 100 MHz 4.5V Displays zeros
TP4 400 MHz 4.5V Displays zeros
TP4 500 MHz 4.9V Displays frequency
TP4 900 MHz 4.9V Displays frequency


TP11 No input signal 0.11V

Same voltage across all frequencies Displays zeros until it gets a 500 MHz input, with which, and up at 900 MHz the counter displays the frequency.

My test:

TP2 30 mV input signal @ 10 MHz 3.4 / 3.5V displays zeros

Warm replacement Q6 with tip of iron, momentarily.
4.9V and displays frequency

On Band 3 with no input signals to the counter TP2 shows 4.9V

On Band 2 with no input signals to the counter TP2 shows 3.4 / 3.5 V (wavers about between the two).

Hope this helps, thanks Jeremy, I was sure replacement Q6 would change *something*
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Old 4th Dec 2012, 6:40 pm   #33
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Default Re: EIP 545A 18GHz counter questions. Signature analysers?

I've just got in from work and I'll have a look through your results later.

However, here's my first thoughts...

I'm not that surprised Q6 is proved OK as it should have an easy life in terms of reliability as it operates at low voltage and only switches a few mA and it has large resistors in the collector and base. So it ought to be fairly bulletproof.

What is a bit odd about Q6 is the choice of bias resistors to bias it for the DC current into the mixer. The 750R and 11k and 8k2 seem a bit marginal in terms of defining a definite OFF characteristic when driven by the PIA.

Therefore, this circuit will probably misbehave if you heat it when the PIA spits out a logic 1 at TP2. This is because a base emitter PN junction typically shifts its threshold voltage with temperature.

So if you heat it up to 100degC from 20degC then this may be enough to flip the state of Q6 because of the odd choices for the bias resistors.

So maybe this circuit behaviour is normal (as in it wasn't ever expecting to see a soldering iron LOL) but what is interesting is 'why' this helps the counter to progress to achieving lock.

Basically a healthy counter will go through a set routine as soon as it gets a flag from that input detector diode when you inject a big enough test signal at the range 2 input. it sounds like 'something' is halting the routine but you are forcing the routine to change/progress by forcing a change at Q6 with the soldering iron.

So I guess we have to work out why temporarily flipping the logic state at Q6 gives it a useful legup...

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Old 4th Dec 2012, 7:23 pm   #34
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Default Re: EIP 545A 18GHz counter questions. Signature analysers?

If it helps I can try and get the voltages on the legs of Q6? I am sure I am not raising the temp anywhere near 100C, neither surface, or even more certainly, the core temperature. I will try and measure the bias resistor values, but the board is densely packed in things region, double sided, and a real fiddle to safely remove things, or even just legs of things, without risk of damage. I am 99.9% certain it's Q6 that reacts, I can literally freeze, (as in a frost on them), all the components around Q6 and only warming Q6 itself causes the change. Could the bias resistor values having changed cause this?

Bear in mind ramming a LOT of signal in (2V), sometimes has a similar effect, but not anything like 100% reliably, and not at all when the whole thing's stone cold.
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Old 4th Dec 2012, 8:18 pm   #35
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Default Re: EIP 545A 18GHz counter questions. Signature analysers?

I AM A FOOL, really sorry, see below:

I have been working off a schematic that's very blurry and struggling to read the device numbering. I have just realised the version I posted a link to is different and a lot clearer, although seemingly of the same manual. I found two or three copies of the manual on the net, and downloaded all of them. All the time I have been referring to Q6 it is actually Q12.... I can see the numbering much better in the other manual.

I am REALLY sorry, but some of the components are the same value around it, and it's in a similar looking bit of circuit, at least to my uneducated eyes... I feel, and am, a right pratt, so Q6 is really Q12.... So Q12 responds to heat, and it's Q12 I have renewed. I have deleted the blurry copy and am working off the clearer one I linked to. Come back paper manuals... I have wasted your valuable time with duff info, I can only apologise.
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Old 4th Dec 2012, 8:51 pm   #36
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Default Re: EIP 545A 18GHz counter questions. Signature analysers?

Q12 voltages

E 12.07V no input signal : 12.08V with 980 MHz @ 50mV frequency is shown

B 11.66V no input signal : 11.68V with 980 MHz @ 50 mV frequency is shown

E 0.22V no input signal : 0.0V with 980 MHz @ 50mV frequency is shown

E 0.22 V no input signal : 0.22V with 10 MHz @ 50mV no frequency shown (zeros)

Warm Q12 and emitter voltage starts rising instantly and around 0.52V the counter shows 10 MHz.

Keep warming a bit more and emitter voltage rises as high as 11.9V

As Q12 cools voltage smoothly falls and counter stops showing 10 MHz frequency at about 0.52V and shows zeros.

A minute or so after warming emitter voltage is back down to about 0.22V with the 10 MHz input signal @ 50mV and counter still shows no frequency (zeros).
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Old 4th Dec 2012, 10:01 pm   #37
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Default Re: EIP 545A 18GHz counter questions. Signature analysers?

OK this makes a lot more sense wrt the symptoms and also the flowchart

From looking at your TP4 results it looks to me like the PIA isn't controlling Q12 correctly. This prevents the counter from seeing signals below about 185MHz as Q12 is meant to bias mixer MX1 as a switch on the lower ranges of range 2. The PIA is presumably meant to pull TP4 to 0V when trying to see frequencies in the 10-185MHz range? This would bias Q12 on and give about 11V at the collector of Q12 to bias the mixer IF port into conduction.

Note that the MX1 TFM-2 IF port acts as the LO port in this circuit.

Can you look to see if pin 15 of the big 40 pin MC68B21P (U2) is connected securely? i.e. is it a dry joint or is this chip in a socket? If so is the pin making good contact with the socket? This is the pin that connects to TP4.

Otherwise the PIA could be duff or there's a system issue that prevents normal control of this PIA.

Note that you have the B version of the 6821. i.e. 68B21 and this is the faster (2MHz?) version.

Last edited by G0HZU_JMR; 4th Dec 2012 at 10:15 pm.
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Old 4th Dec 2012, 10:28 pm   #38
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Default Re: EIP 545A 18GHz counter questions. Signature analysers?

Note also that Q12 is a very similar circuit to Q6 and will therefore have similar issues with thermal stability as it uses a similar bias network.

The network makes the on/off switching threshold for the transistor very vulnerable to changes in temperature.

So I'm not totally surprised that Q12 or Q6 can be turned on using heat from a soldering iron. What is surprising is why the threshold is so fuzzy by design?

Each PIA port pin on ports A and B of U2 has three states:

1/ it can be programmed at will by the processor to act as tristate (Hi impedance) port pin

2/ it can be programmed at will by the processor to act as an output port pin with logic 1

3/ it can be programmed at will by the processor to act as an output port pin with logic 0

I would assume the counter just employs the last two states and switches between the logic levels as required?

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Old 4th Dec 2012, 10:28 pm   #39
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Default Re: EIP 545A 18GHz counter questions. Signature analysers?

It is the B version, and it is in a socket. I have removed it, checked all pins are clean, used contact cleaner and reseated it twice. I have re soldered the socket pin to the PCB, and all joints off that track. Sadly no change is apparent. What's R57? Says 2% S.A.T

It seems a selective value on the schematic 4.3K NOM (nominal?)

Any way to test the relevant part of U2 further?

Another naive question, is this MC68B21P programmed in any way, or can it just be replaced with a new one, they seem available on the well known site.

And I meant collector voltage, not emitter....I'm hopeless, a bit stressed with work this week, mad rush pre Christmas.

Thanks Jeremy.
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Old 4th Dec 2012, 10:40 pm   #40
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Default Re: EIP 545A 18GHz counter questions. Signature analysers?

If you are feeling brave you could swap U2 with another 68B21 used elsewhere in the counter if they are in sockets?
eg the one used as U7 for the A108 board would be a good choice. This selects the N code for the VCO PLL.

NOTE: I'd avoid swapping for the YIG PIA in case it gets programmed to a high YIG current with a duff PIA fitted.

U7 is much safer to swap for as you will just get a duff N code and lose PLL lock on the upper parts of range 2 if the PIA turns out to be duff on pin 15.


Then see if pin 15 (TP4) ever goes to logic 0 on the range 2 PIA for 10MHz inputs to range 2?

Otherwise you could buy a MC68B21 from ebay for about a tenner. I've only got the 1MHz versions here sadly...(loads of them!)

Quote:
What's R57? Says 2% S.A.T
R57 presumably acts as a current limiting resistor into the mixer IF port. I'd expect it to be maybe a few kiloOhm in value to set the mixer diode bias current to a few milliamps from the 11V available from Q12 collector when biased on? Q12 is configured a bit like a crude current source so I'm not sure why R57 is marked as a SAT part?

However, they may have been adjusting R57 during factory testing in order to control the degree of insertion loss of the mixer (across 10-200MHz) when biased as a thru switch via Q12.

There's something a bit odd about their choice of bias resistors around Q6 and Q12 and I think I'm missing a trick here. Maybe there are three control states for this circuit rather than two?

Last edited by G0HZU_JMR; 4th Dec 2012 at 11:08 pm.
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