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Old 14th Mar 2017, 1:33 pm   #1
Specmaster
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Default Frequency counters

Hi Everyone, I have an old but serviceable Tech TE-20D signal generator with a vernier tuning dial which by definition is not particularly accurate.

I'm planning on adding a digital display that I can mount on the top of the unit similar to the one shown on the attached photo. I'll be mounting an BNC connector on the rear of the signal gene and feeding this directly from the top of the RF attenuator pot so that the counter is always going to get a good signal regardless of the position of the attenuator pot.

My question is, has anyone used one of these counters before and tell just how accurate they are, I reckon they have to be far better then the current vernier scale system?
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Old 14th Mar 2017, 2:07 pm   #2
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Default Re: Frequency counters

They're usually within a few Hz or so of reality. Depends on the crystal quality really.

The main problem is they will load the generator terribly, particularly at higher frequencies and aren't very sensitive.

TBH I'd hang around eBay etc. and grab a second hand Racal/Black Star/Thandar counter of some description. I've picked them up for less than 20 and they tend to have better sensitivity (down to 5mV), lower loading (1Mohm) etc. Plus if you know someone with a frequency standard or another accurate counter, you can compare it to that and adjust it. It might be 4-6x the price but it's 100x as good.
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Old 14th Mar 2017, 5:48 pm   #3
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Default Re: Frequency counters

The sensitivity is often 100mV or worse, but this device shouldn't load the output too much if it's taken across the 1K pot. And the signal level quoted for the TE-20D is 100mV! You might be OK.
The loading effect is much more noticeable if you try to "sniff" the local oscillator on a comms receiver. then the loading can change the frequency. For my HRO I had to be careful to place a pickup cable near enough to the grid cap to get a good signal, but not too close to cause frequency shift.
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Old 14th Mar 2017, 6:01 pm   #4
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Default Re: Frequency counters

Funnily enough I was watching this video last night. He has the same idea and issue as you and adds a buffer circuit to one of those Tenma/Leader sig gens. Or in fact he doesn't in the end as it worked OK but the circuits clearly visible in the video.

https://www.youtube.com/watch?v=3Mgg8wvyDHw&t=3090s

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Old 14th Mar 2017, 6:03 pm   #5
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Default Re: Frequency counters

Considering how cheap these are, they're truly spectacularly accurate [to within a few tens of parts-per-million, worst-case!]

If you're worried about loading of the circuit you connect them to, a simple JFET source-follower

http://www.nutsvolts.com/uploads/wyg...rston_Fig6.jpg

will give you a seriously-high input impedance - good-enough that a few inches of insulated wire stuffed into the signal-frequency side of your existing oscillator will 'sniff' enough RF to give a good count.
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Old 15th Mar 2017, 12:23 pm   #6
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Default Re: Frequency counters

For this JFET source follower, would 1/4 watt resistors be sufficient for the values shown?
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Old 15th Mar 2017, 4:28 pm   #7
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Default Re: Frequency counters

Oh yes.
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Old 20th Mar 2017, 1:40 pm   #8
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Default Re: Frequency counters

I've built the JFET source follower and if anything it is actually worse now. The maximum output from it is 5mv half wave with the signal generator set at maximum output. Ok, I don't as yet have the frequency counter as shown in the first post, still on-route, but I do have one built into my function generator MHS-5200A which I have been using.

With the supply set at 12v it gives 3mv and cranking the supply to 20v gives 5mv output and I suspect that the MHS-5200A demands all of that especially at FM and above frequencies.

I have a Black Star Meteor 100 arriving soon but even that needs 10mv, can this low level signal be further amplified to produce something in the realms of 20mv or so and can anyone suggest a circuit that I could incorporate with the source follower to allow the counter to operate correctly?
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Old 20th Mar 2017, 2:02 pm   #9
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Default Re: Frequency counters

Hello,
if you need amplified signal- build please a plus transistor stage_i.e. as suggested in Andrew`s video at 21:00, it has a gain of cca 10...
Regards, Karl
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Old 20th Mar 2017, 3:49 pm   #10
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Default Re: Frequency counters

Quote:
Originally Posted by Specmaster View Post
I've built the JFET source follower and if anything it is actually worse now. The maximum output from it is 5mv half wave with the signal generator set at maximum output.
How are you measuring the output voltage? I.e. what is the hookup, frequency, measuring equipment?
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Old 20th Mar 2017, 5:35 pm   #11
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Default Re: Frequency counters

It was measured on a Watsu SS-5710 scope with a 1M input impedance.
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Old 20th Mar 2017, 8:38 pm   #12
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Default Re: Frequency counters

Ok that shouldn't load it heavily. I looked at the manual and the "high" level output jack should be max 100mV.

Are you using a 10x or 1x probe?
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Old 20th Mar 2017, 9:40 pm   #13
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Default Re: Frequency counters

The measurement was done with the probe set to 1x, otherwise the signal was too low to achieve a trigger lock to enable a proper measurement to be taken.
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Old 20th Mar 2017, 11:44 pm   #14
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Default Re: Frequency counters

This thread seems to be going over some similar ground to Skywave's thread(s) a few months back on RF Voltmeters e.g. http://www.vintage-radio.net/forum/s...d.php?t=131879.

A number of posts in that thread seemed to suggest that the simple J-FET buffer amps (pubslished in numerous places and often using 2N3819 or MPF102) were often found to be very good attenuators! I think Skywave ended up designing a very good RF amp with a 2N4416 FET and a pair of BRF96's (?) that gave a useful gain that was flat over a very wide frequency range. Could be useful here I guess.

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Old 20th Mar 2017, 11:47 pm   #15
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Default Re: Frequency counters

Just did some more measurements on the scope direct from the low and high outputs of my sig gen, (Tech TE-20D) and obtained the following:
At 130Mhz Low is 7mv and high is 1.8V probes on 1x
At 120Khz Low is 40mv and high is 3.2V probes on 1x.

I think that maybe my Tech has gone tech as the spec for it states that the Low has a maximum of 100uv and high of 100,000uv but it does not state the frequencies these were measured at?
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Old 21st Mar 2017, 12:05 am   #16
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Default Re: Frequency counters

At 1x your probe bandwidth isn't likely to be very high. This will be knocking the reading off at 130MHz. Might be worth trying to trigger on high when measuring low and vice versa.
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Old 21st Mar 2017, 2:05 am   #17
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Default Re: Frequency counters

Hi Specmaster,
tyical 1x probe bandwith is i.e. 30MHz what has an attenuation for 130MHz min 12dB (calculated with 6dB/octave roll-of), possibly at 15dB... Btw; is it correct compensated by calibration source please?
Regards, Karl

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Old 21st Mar 2017, 4:09 am   #18
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Default Re: Frequency counters

Quote:
Originally Posted by Specmaster View Post
I think that maybe my Tech has gone tech as the spec for it states that the Low has a maximum of 100uv and high of 100,000uv but it does not state the frequencies these were measured at?
Looking at pictures of that sig gen on Google, it looks as if it has very little internal screening, so maybe not too surprising if it is leaking RF out.

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Old 21st Mar 2017, 5:12 pm   #19
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Default Re: Frequency counters

Update, I was trying to read output voltage at 130Mhz on a scope that only went up to 60Mhz, a senior moment I think. Remeasured again on a 100Mhz scope and it seems that its chucking out about 600mv at 100Mhz and around 3 volts pp at 116Khz on the high output jacks.

To be honest I'm amazed that the 60Mhz scope was still able to give me a reasonable trace so far beyond its range, maybe it was harmonics?

I read Skywaves thread regarding this sig gen and I think I'll need to check the caps before I go much further as my sine waves are badly distorted on the lower bands.
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Old 21st Mar 2017, 6:24 pm   #20
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Default Re: Frequency counters

The "JFET Source-follower" does work - I've used it extensively. You need to take care to accept and acknowledge the wide variability in individual JFET pinch-off characteristics and adjust the bias accordingly.

[There's something to be said for lifting the earthy-end of the gate-to-ground resistor and feeding it from a potential-divider across the supply rails: then adjust this potential-divider to get approximately half the DC supply-voltage at the source]
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