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Old 12th Feb 2018, 11:50 pm   #21
WaveyDipole
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Default Re: Function generator with sweep function

I'm only able to get an impression about the PM5324 from the online photographs which don't always give a good indication of scale so thanks for pointing that out.

I have a Heathkit RF1U valve signal generator, purchased from the forum, which is naturally analog and all knobs. The dial seems pretty accurate although there is no sweep, it does sine only, tends to drift a bit and the audio tone is a bit quiet. However I have found it quite convenient to use for aligning radios. Thinking about your earlier suggestion, it would be much easier to sweep with this than with the Thandar TG2001 and since adjustment is via a variable air spaced 'tuning' cap, there is no worry about wearing out a pot. I therefore do kind of get the point about speed and convenience of operation with knobs.

I appreciate the various viewpoints and experiences given here and will take them into account when making a purchase decision. For now, I think, its a matter of sitting tight and waiting for something to come up, meanwhile seeing what I can do with some experimentation including following the suggestions given.

Last edited by WaveyDipole; 12th Feb 2018 at 11:59 pm.
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Old 13th Feb 2018, 3:53 pm   #22
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Default Re: Function generator with sweep function

Dear WD,
if you wish to test only RF-receivers, you will need sweep levels in range of 1uVs-maybe-upto-1mV, but suhrly not over 10-100mV!! When you have to test filters i.e. you can do it at tens of mVs level or even at1 V, but I mean its an abs. maximum_ so its uninteressant for your application if an generator delivers level over 1 or even 10V RF output_their are to first of interest by EMC tests_I think...
I belive its a very nice UK produced generator, the TF2016, what you can take for your search list too_its true analog AM-FM RF-signal generator in F-range of 10K-120MHz, nice compact, full transistorized-easy to have spares, prices are 100-200$_but relative rare. For audio tests you will need some what others, (Wien-Bridge/Audio oscis_OK, funct-gen too) so is for me an(often plastic cased) function gen not to take for your laboratory needs (because theire delivers eventually more strayed signal as you need to the inputs as test-signal!)_ in addition you have to organize an attenuator up to cca 80dB too_but theire are only my subjective aspects.
Rgds, Karl

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Old 13th Feb 2018, 4:49 pm   #23
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Default Re: Function generator with sweep function

How about an ICL8038-based generator? If you set the duty cycle to give a ramp waveform from the sawtooth output, the square wave becomes a 'scope sync pulse at the same time (no pun intended). You could use that with your existing signal source.

Ready-made boards are available from eBay suppliers based in .cn with lead+delivery time around 2-3 weeks (I've just PM'd you the specific link for the ones I bought recently)

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Old 13th Feb 2018, 9:58 pm   #24
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Default Re: Function generator with sweep function

The TF2016 has been noted. Thanks.

Interesting idea about the ICL8038. For a couple of quid its perhaps worth having a play with. I have also come across this ramp generator circuit by w2aew which is not too difficult to build:

https://youtu.be/bVahBmsvirE

I have most of the bits lying around with the exception of the pots. I might give both ideas a go.
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Old 13th Feb 2018, 10:04 pm   #25
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Default Re: Function generator with sweep function

Never thought of that, which is silly because I built a ramp generator here as well which would work:

https://www.vintage-radio.net/forum/...1&postcount=39

Add summing amplifier for offset and gain control and you're done.
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Old 17th Feb 2018, 10:31 pm   #26
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Default Re: Function generator with sweep function

Just tripped over an interesting project which might cover this requirement:

http://www.intio.or.jp/jf10zl/csg.htm

I'm actually considering building one. I reckon you can substitute most of the parts for junk box parts. 36v zener works pretty well as a varactor too
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Old 17th Feb 2018, 10:56 pm   #27
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Default Re: Function generator with sweep function

In the linked item, I like the "Radder crystal filter" -- I know pronunciation of "L" ends up "R", but surely it can be typed correctly?
I first came across this amusing confusion in a CD player technical manual about 28 years ago. It took me a while to understand what was meant by "Read" instead of "Lead" (or similar reasonable mix ups which could send you off at a tangent).
Les.
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Old 18th Feb 2018, 10:11 am   #28
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Default Re: Function generator with sweep function

Without diverging too far from the topic, Japanese is such a terrifically different language to ours that I'm impressed the author managed to get as far as he did. I have found it helps if you read translated stuff in the native accent in your head. Sometimes it's like re-enacting Allo Allo in your head though
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Old 26th Feb 2018, 4:48 pm   #29
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Default Re: Function generator with sweep function

Can anyone give me a view on this one please:

https://www.ebay.co.uk/itm/Two-HP332...gAAOSwhcdakI9y

I'm tempted, but a little concerned about the 'been got at' likelihood. What's the significance/impact of what he states here:

Quote:
It has one little issue.The relay panel doesn't respond not switching.
I wired the output direct from the PCB to the BNC connector on the front of generator.
The other one sounds like it has been stripped down. Is there still any value in it for spares? I.e. are the parts that are most likely to fail the ones he mentions that have already been stripped out?
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Old 26th Feb 2018, 5:15 pm   #30
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Default Re: Function generator with sweep function

Not enough knobs!

Seriously, I wouldn't. Not just because of the early cautions from RW, but the fact it's massive! Those examples ring too many alarm bells for my liking. It sounds like the output attenuator is toast on the working one - that might be a simple fix, but if someone has done something daft with the output, it might not be.

If you fancy HP, the 3314A I mentioned earlier is a better bet as it at least has 1 knob! It's a conventional function generator with digital control, so it won't be as stable as a DDS-style generator - not that I've ever found a time when that matters. I've been trying to use it more since my earlier post, and am getting used to it. It does AM and FM pretty well. It also has an arb mode, but setting that up from the front panel isn't terribly realistic, and I'm not sure of its value TBH.
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Old 26th Feb 2018, 5:27 pm   #31
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Default Re: Function generator with sweep function

That's very expensive for two 3325's as well. I just sold my Rigol generator for £280 to fund an RF signal generator. That's nearly in that territory.

Another option I found is the QRP labs VFO kit.

http://www.qrp-labs.com/vfo.html

They have an application note which describes the construction of a low distortion signal generator using their modules: http://www.qrp-labs.com/images/appnotes/AN006_A4.pdf

You won't get a swept display as far as I can see but using a diode probe at the other side of the IF chain you could manually sweep and peak it.

Edit: if you look at the RF power meter I posted in the homebrew section, I suppose you could attach a Si5351A DDS module to the remaining ports and knock up some sweep software pretty trivially. Might do that!
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Old 26th Feb 2018, 7:23 pm   #32
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Default Re: Function generator with sweep function

I tried an XR2206 DDS Function Signal Generator as a sweep generator by connecting the sweep output (+/- 10V) of a Scopex 4D 10A to the fine frequency potentiometer on the DDS Function Generator though a 330 kΩ resistor. For good measure the output of the DDS Function Generator was also amplitude modulated by disconnecting pin 1 of the XR2206 IC from ground and connecting it to the square wave output of an AN8008 multimeter through a 20kΩ resistor. Two more resistors connected to pin 1 and ground and supply were needed to suitably bias pin 1. The AN8008 allows selection of a range of modulation frequencies between 50Hz and 5000 Hz.

The upper trace in the photo is the modulated and swept output of the XR2206 DDS Function Signal Generator roughly centred on the IF frequency of 465 kHz. The lower trace is the signal detected in the wireless. The slight asymmetry is due to the AGC action. The asymmetry became more pronounced at a slower sweep rate. The instructions for the wireless set suggest connecting the negative of a 9 V battery to the AGC rail when aligning the IF but I had not done this.

This was only a quick trial but it may be possible to develop the XR2206 DDS Function Signal Generator into a wobbulator subject to a maximum frequency of 1 MHz. In this case I would change the timing capacitor from 100 pF ceramic to 1000 pF mica as 1000 pF is the minimum specified in the XR-2206 data sheet.

David
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Old 26th Feb 2018, 7:41 pm   #33
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Default Re: Function generator with sweep function

Thanks gents. This is pretty much as suspected and I forgot about that warning from RW, so thanks for the reminder. I am glad I checked first and that idea is in the bin.

I do have a plan in motion for a DIY approach, even if for my own education, although I did not think about using the XR2206 IC that I have knocking around. I will have a think about that.

In the meantime, if something useful presents itself then I would still be interested. For some reason the HP 3314A was not on my list although I was sure that I had added it. Well, it is on there now. I have also made a note to avoid the HP3325A, remarking on the comments made by RW and its size.

The Si5351A Synth module kit looks like an interesting project although it doesn't cover below 3MHz so it would not be able to sweep a 455 or 470kHz IF. There was an offer of a Taylor 62 MkII on here the other day, but the same problem, only 4MHz upwards. I might have been tempted by that Rigol however.... I'm curious why you sold it Mr Bungle? Was it the points about Function Gen vs proper RF gen made earlier this thread?

Last edited by WaveyDipole; 26th Feb 2018 at 7:46 pm.
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Old 26th Feb 2018, 8:28 pm   #34
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Default Re: Function generator with sweep function

The keys on the 3325A and a lot of HP gear of that ear are plastic caps clicked onto a custom made plastic mechanism with a little spring steel clicker. It pushes gold plated beryllium bronze contacts to short a pair of gold pads on the board

They're called "Bill West Switches" after their designer.

When they're working, they're OK. Eventually the gold wears off on the board. If the plastic mechanism breaks, they aren't obtainable, and their mounting lugs pass through the board and are melted to hold them. Second hand ones are ruined if removed.. You can add alternative little 'tact' switches but the keycaps won't fit. A job for a high resolution 3D printer?

3325A isn't an RF sig gen, so the output voltage doesn't go low enough for radio work. The effort went into making square and triangle wave shapes as a general electronics lab thing.

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Old 2nd Mar 2018, 9:54 pm   #35
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Well I have moved forward a little bit with this. The recent inclement weather has given me a bit of time to sit and put together a project based on the W2AEW circuit. I select this because the sweep frequency, gain and DC offset can all be adjusted and the circuit uses parts that are easy to obtain. The original circuit is powered by two 9v batteries, but I already had a left over project box with a 12-0-12v power supply. Fortunately both ICs can operate at much higher voltage, so all that was needed to make the circuit operate at 12-0-12v was to tweak of some of the bias resistor values.

Further modifications included adding a 10k preset to allow adjustment of the trigger level and further 10k preset to set the 'selected' resistance value marked with a '*'. A small 47nF capacitor was added to the inverted input of the first op amp stage to improve stability as the SPICE simulator was showing periodic spikes under certain circumstances. I tested all of the modifications in a SPICE simulator along with four different timing capacitor values (220uF, 22uF, 2.2uF and 220nF). Once testing was done to satisfaction I started work on construction of the actual circuit. I used a 4 way rotary switch to select the 4 capacitors to allow for the 4 sweep speed ranges. The only bits I had to buy were the 3 pots and a couple of resistor values I did not have to hand. After a couple of hours work I had a working ramp generator circuit.

The result seems to work pretty much as expected. I have found that if I start with the offset set to zero on the sweep generator and the centre frequency set on the signal generator, I can then use the gain control to set the sweep width. Triggering on the sync pulse fed into another channel I can adjust the timebase control to set the sweep interval to correspond to the exact width of the 10 graticules on the screen and the target frequency is then dead centre. It is also possible to adjust this by varying the sweep speed. Start and stop frequencies can be obtained by manually measuring the voltages required to generate those frequencies and the setting generator to the start frequency and the ramp height to the voltage difference.

I did a brief test on an IF choke and the result is in the attached picture as well as the almost finished project box. Somewhere (in a 'safe place') I have the other 3 knobs... I did notice that I needed to leave the earth clip from the generator output disconnected from the input side of the choke otherwise the response was almost flat. Not sure why that is, unless it has something to do with input impedance matching? I will experiment some more in the days to come.

In the meantime I have also been offered a signal generator with a sweep function by a member. I had already started my project before the offer came in, so I decided to see it though and it has provided a bit of fun while being educational. However, I do hope to be able to take up the offer in the near future as this is a professional tool which will take much of the guesswork out of the equation.
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Old 3rd Mar 2018, 5:23 pm   #36
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Default Re: Function generator with sweep function

Nice work, W/D!
Guy
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Old 3rd Mar 2018, 5:54 pm   #37
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Default Re: Function generator with sweep function

Indeed. Looking good!
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Old 20th Mar 2018, 9:49 pm   #38
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Default Re: Function generator with sweep function

Thanks. Finally discovered the missing knobs in their "safe place" and added some legends.

I recently tried using it in conjunction with the sig gen to check the spare R300 chassis which didn't quite sound right to me using this sweep method. It does take a few moments to set things up on the oscilloscope once everything has warmed up, but once done, the trace seems stable and I have found it easier to visually find the misaligned choke and correct it. Setting up the target centre frequency does seem to be a bit of a guessing game though. I know I am pretty close as the peaks of each of the chokes were no more than half a graticule from each other and within half a graticule of the centre of the scope screen. I can use a cursor to mark the one which looks closest to the target frequency and tweak the others to match so its possible to get them all aligned to each each other, but I don't know for sure how close I am to the target frequency of 455kHz. Here is where a signal generator with digital readout and precise controls would no doubt be much more accurate. Still, this sweep method does work and seems a bit more accurate and reliable than listening for the loudest point or using a DMM, which is what I was doing prior to this. The chassis did sound rather better and sensitivity appeared to be increased after the tweak. There is still a problem with the AGC I think as the volume is still a little low and I am wondering whether the AGC is clamping down excessively due to the higher gain of the GT322 transistors that replaced all of the original AF117s?
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Old 20th Mar 2018, 10:50 pm   #39
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Default Re: Function generator with sweep function

If you add a pot (or any variable voltage source) to the design which you can switch in instead of the ramp generator as an option, that can be used to indicate the centre frequency. Connect a counter to the TTL output on the generator and move the pot until it outputs 455KHz on the counter, then align the scope X position with the centre of the graticule. You can also use that to measure Hz/div etc.
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Old 20th Mar 2018, 11:40 pm   #40
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Default Re: Function generator with sweep function

Yes, that's effectively what I do with my Wavetek 162 that drives the RF oscillator. Adjust the sensitivity of the channel driving the X plates to get whatever you need (e.g. 4kHz per horizontal division for an AM set). Switch back to the oscillator when you're done.

To implement that on this circuit, I reckon you just need to disconnect the 555 from the op-amps (you might need to connect it to a fixed DC voltage from a couple of resistors - perhaps a pair of equal ones to give Vcc/2, given that the ramp goes from 1/3 to 2/3 of Vcc). You have the DC offset control already, so no need to buy another pot and knob. A toggle switch on the front panel would be easy to add. Just a thought

Nice job, BTW. Sorry I missed this earlier.
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