Cheap HF signal source 5-16MHz at least - Page 1
Forum section: electronics / testgear

author: setq
author_rank: Frequent Contributor
timestamp: October 11, 2016, 10:15:05 am
content: I need a signal source that spans 5-16MHz with AM/FM modulation to test a few RF things I'm working on. I've been struggling along with an old analogue function generator with a 2MHz ceiling and a hacked up VFO which is very linear but unstable as feck and obviously not calibrated. Also frequency sweeps are inconvenient as anything with it as it has no reduction drive. Something that sweeps by itself would be nice! Plus I have no AM or FM modulation support on it other than injecting the function generator signal into the leg of the emitter follower amp in it. This is all crappy, inconvenient and difficult to control.What cheap (under £100) options do I have to solve this problem?The ubiquitous FY3224 DDS generators on ebay seem to fit the bill but I suspect they are junk. Ideally I'd like something Agilent like a 33120A would be nice but they don't exactly grow on trees in the UK.Something that allows me to get rid of the function generator and the VFO would be nice so nothing specific to RF and calibrated in dBm etc. That would be a PITA.

author: ebclr
author_rank: Super Contributor
timestamp: October 11, 2016, 10:48:41 am
content: I bought this one and is amazinghttp://www.saelig.com/product/sdg2042x.htm

author: CJay
author_rank: Super Contributor
timestamp: October 11, 2016, 11:32:14 am
content: If you can program a PIC then there is firmware out there that will control one of the cheap AD9850 DDS modules for sweep but speed is an issue. AM modulation is simple to do, again there are examples out there using those DDS modules but FM is much more difficult (not impossible), you'd need a variable attenuator and some way to measure the output but again, that's possible to homebrew, especially at HF.You could put together a workable sig gen for under £25 if time is free (count it as such for a hobby project).

author: setq
author_rank: Frequent Contributor
timestamp: October 11, 2016, 01:48:52 pm
content: FM is aspirational so I can possibly do without that. I hadn't looked at the DDS modules actually; will head in that direction. I did some time with the old PIC16 architecture in the dark ages so no problems with that. This is purely leisure so the cost vs time trade off is in favour of time.Thanks for the suggestion.I would love something a little more high end but I can't justify it.

author: Alex Nikitin
author_rank: Super Contributor
timestamp: October 11, 2016, 02:06:09 pm
content: Quote from: setq on October 11, 2016, 01:48:52 pmThere are some cheap RF signal generators on eBay UK. Few years ago I've bought a proper Marconi 2022E for about £100, not fully working but it was an easy fix. I've looked right now and there are some inexpensive gear available, starting at about a fiver for an old broken Heathkit RF gen.CheersAlex

author: setq
author_rank: Frequent Contributor
timestamp: October 11, 2016, 02:22:35 pm
content: I've got a dead Marconi 2019A for ref. Paid £10 for it. It did work but the attenuators, power supply and displays are shot now. Plus I did my shoulder in shifting it and it's in my bad books for this so I'm just harvesting it for parts periodically. If I find another dead one for cheap I'll probably end up with one working one. Lovely bit of kit but useless as a gen purpose signal source.A Heathkit RF gen is what I based my VFO on. I also considered a varactor based sig gen but I can't seem to get a decent tuning range out of one. I could probably get away with 1960s tech for this really.

author: CJay
author_rank: Super Contributor
timestamp: October 11, 2016, 03:00:58 pm
content: The code for DDS chips is already out there so it's just a case of selecting the code you want and writing it to the chip. If you were really stuck for a PICKit2, post me a chip and I could write a hex file to it for you.The other alternative would be a MAX038 chip, which offers the possibility of FM modulation.Where in the UK are you? Might it be worth joining your local hackerspace or Ham club to see if there are members who have this gear?

author: barry14
author_rank: Regular Contributor
timestamp: October 12, 2016, 12:06:51 am
content: You can get an HP 3312 Function General for under $100 on Ebay. It has a modulation generator built in and will do both AM and FM (externally also). It has a 50 ohm output impedance and an accurate 20 dB step attenuator. It is limited, however, to a top frequency of 13 MHz.

author: oldway
author_rank: Super Contributor
timestamp: October 12, 2016, 07:00:44 am
content: I have an old valve HF generator I buy very cheap: a Leader Sweemar LSG-532.The marker generator is very stable.http://www.miniwatt.info/leader-lsg532.htm

author: nfmax
author_rank: Super Contributor
timestamp: October 12, 2016, 08:54:33 am
content: Quote from: barry14 on October 12, 2016, 12:06:51 amYou are unlikely to find one of these in the UK for that sort of money! Anyway, it is a classic integrator/comparator function generator and nowhere near stable enough for the required purpose (though they are very nice). Most cheap DDS-based function generators will have adequate frequency stability, although they often have poor phase noise & spurious issues. Hardly any of them have an output attenuator which is up to the job, though you can always use an external attenuator. However a decent switchable RF attenuator is likely to £50 or so on UK eBay

author: setq
author_rank: Frequent Contributor
timestamp: October 12, 2016, 09:19:18 am
content: Indeed. I haven't seen something suitable go for a reasonable price. Stewart of Reading are selling a suitable HP one for £395+courier+VAT and that's cheaper than ebay options for something decent.Big thanks to everyone.Some updates on where I am with this. I've taken things into account that people have suggested here. Current status quo, is this is what I'm using for a signal source (cranky looking thing but it works!):This is buffered by half of this bidirectional amp. The other half is blown up due to an accident:This is then attenuated by a pi network hacked up by hand as required. This is terminated in this dummy load (it's a bit more complicated than that but fundamentally it works as the CF resistor inductance isn't terrible:And a BFO that has been slightly pulled is the other generator:Plan going forward is to do the following as I've done a nice little spreadsheet of all things.1. Attempt to use pn2222 reverse biased B-C junction as varactor diodes, possibly a few stacked up and see where I get with that as I have a ton of them floating around so can select. If that gives a reasonable tuning range and isn't too non-linear, build a new colpitts oscillator around that buffered with amp/attenuator/amp/attenuator and try and hit 50 ohms impedance and do two frequency ranges with a switched inductor. I'll add gain control to the last amp which can be modulated by my current function generator. I need to try the cheap option first as I might learn something doing it. Oh and varactor voltage control can be used to FM modulate it as well as set the initial tuning avoiding having to obtain another air variable capacitor.2. If that fails to give adequate tuning range, get an AD9850 DDS board in and get that to talk to an AVR. I'll write some custom code with C for this (day job is software so no problems). I need to research this thoroughly. I know how DDS works, just not how to process the synthesized output properly.3. If that fails, buy an HP generator for around £500 and hide it from the wifeI'll keep an eye out for Heathkit generators too. Whilst not "lab grade" they might do for filter sweeps by hand if can can cal them against a frequency counter.There's a design for a switched attenuator front end for the "poor man's SA" so I'm probably going to rip that offAlso have a 74hc4046 PLL based VCO on a breadboard but it's unstable as hell at the moment so I might abandon that one until I have some clue as to what the hell I'm doing.

author: nfmax
author_rank: Super Contributor
timestamp: October 12, 2016, 10:16:39 am
content: A bit of Googling turned up this:http://www.zl2pd.com/ptovfo.htmlwhich may give you some ideas. Certainly looks cheap enough, I love the idea of a 'plug-tuned oscillator'!(Edit to add) And on the same website, this one:http://www.zl2pd.com/SingleSpanOsc.html

author: setq
author_rank: Frequent Contributor
timestamp: October 12, 2016, 10:25:52 am
content: Oh that's rather interesting. Definitely a possibility. I also happen to have some very nice geared motors floating around from something. PID control timeAlso that's a great idea regarding mixing. Allows smaller varactors then running at higher frequency. Happen to have a nice 10 turn bourns pot lying around as well.Thanks for the links.

author: PA0PBZ
author_rank: Super Contributor
timestamp: October 12, 2016, 10:29:32 am
content: http://www.ebay.co.uk/itm/Marconi-FM-AM-Signal-Generator-TF-995A-1-/262668979376They are quite stable, but eat up a lot of desk space...

author: PA0PBZ
author_rank: Super Contributor
timestamp: October 12, 2016, 10:33:26 am
content: More suggestionshttp://www.ebay.co.uk/itm/1-500-MHz-AM-FM-Decade-Controlled-PLL-RF-Signal-Generator-TR-0614-/262669673796http://www.ebay.co.uk/itm/Racal-Dana-9087-signal-generator-/162232371615Edit - oopsQuoteNext time I should read first I guess

author: setq
author_rank: Frequent Contributor
timestamp: October 12, 2016, 10:43:17 am
content: I've given up on that requirement so appreciate the suggestionsdBm/power is more important in RF than voltage/impedance precisely. I've been using that as a crutch to simplify things as I'm primarily building a receiver so high impedance is more important than power transfer. However it's the same banana really.