Using an AM radio as IF+AF stage - where to inject signal? - Page 1
Forum section: electronics / rf-microwave

author: ChristofferB
author_rank: Frequent Contributor
timestamp: April 17, 2019, 11:48:24 pm
content: Hi all, I've gotten this idea that I want to try and modify an AM radio as a generic tunable IF+AF stage for building simple HF receivers with.I'm trying to put an SMA connector on an AM receiver (generic 70s battery transistor radio) to use as IF and AF stages for a HF receiver project,and I'm unsure where to inject the IF signal (i intend to have an AD831 to mix in a BFO for SSB type RX at the input.) It has a typical ferrite rod - should i couple to one end of that through a small capacitor?Should I build in a pi attenuator into the system to dampen the incoming signal?Thanks in advance--Chris

author: innkeeper
author_rank: Unknown
timestamp: April 18, 2019, 12:42:06 am
content: guess that depends a lot on how you are designing it.if you're in need of the crystal filter and IF gain, then do it right after the mixer.

author: ChristofferB
author_rank: Frequent Contributor
timestamp: April 21, 2019, 12:50:41 am
content: Thanks for the idea!Honestly it's just messing about, but it would be interesting to try.I guess I could just throw some loops of wire around the ferrite coil antenna and solder to a coax connector, coupling losely to it.--Chris

author: vk6zgo
author_rank: Super Contributor
timestamp: April 21, 2019, 02:27:34 am
content: The classic way that receiving converters are used is to just use a first IF in the tuning range of the radio it will be used with.The radio can be left fixed tuned to that frequency with the tuning done by the converter,or you can have a tuneable first IF & a fixed tuned converter.In either case, you have a double conversion receiving setup, which has some advantages over just using the existing lower frequency IF.

author: ChristofferB
author_rank: Frequent Contributor
timestamp: April 21, 2019, 10:29:21 pm
content: Quote from: vk6zgo on April 21, 2019, 02:27:34 amThat was my plan, I am just unsure exactly how to inject the signal into the AM radio. But i guess as long as I couple into the tuning LC circuit, either some loops around the main coil or a small series capacitor to one of the terminals on the main variable tuning cap would be good right?thanks for the hints !

author: ChristofferB
author_rank: Frequent Contributor
timestamp: April 26, 2019, 11:51:54 pm
content: So, I ended with something solution-like: 6 loops of wire around the ferrite rod antenna/coil, one side to the SMA chassis, one side, through a series 100 pf capacitor, to the center conductor.It still receives without any input, as expected, but when a 455 khz IF filter is added to the connector, and a lenght of wire, and the radio is positioned near a 455 KHz test signal, I can clearly pick it up as i tune. Granted, the radio might do this unmodified too!Here's an issue: The radio can now be used as the IF stage, but how would one inject a BFO for SSB reception?thanks!

author: vk6zgo
author_rank: Super Contributor
timestamp: April 27, 2019, 03:25:22 am
content: Quote from: ChristofferB on April 26, 2019, 11:51:54 pmInjecting your 455kHz at the original input which is tuned for another frequency range is really making it hard for yourself.It seems you didn't really get what I was suggesting..Let us take an example that you wished to tune to the "80m" Amateur band, which in Oz is 3.5 to 3.8 MHz.( 1 know it is different elsewhere, but it doesn't affect the figures)A MF AM radio tunes from about 500kHz to 1600kHz---- a range of 1.1MHzTo make 3.5MHz appear at 500kHz on the dial, a fixed local oscillator of 3.0MHz needs to be mixed with the incoming "80m band " signals.Calling 3.5MHz f1, the LO , f2, & the original MF band input frequency f3 we have.f3 = f1- f2.Or, for this particular frequency.( quoting all frequencies in MHz):-0.5MHz=3.5MHz-3.0MHz.If we instead wanted to tune to, say, 3.75 MHz we would have ( again, quoting all frequencies in MHz):-0.75MHz =3.75MHz- 3.0MHz.This allows you to tune across that band using the existing MF tuning control.The MF range is then converted to a fixed 455kHz IF because the AM radio's existing local oscillator is variable.The IF passes through several stages, then to an "envelope detector" which demodulates the AM signal.The classic way to receive SSB (or CW) signals is to couple a 455kHz signal into that detector.This will allow you to demodulate your SSB signal quite well, but is not as good as a "product detector", or abalanced mixer.In many cases, just sitting the 455kHz source against the radio will couple enough into the detector to work.

author: ChristofferB
author_rank: Frequent Contributor
timestamp: April 27, 2019, 11:11:54 am
content: Ahh i see, so keep the first external mixer as a fixed downconverter. I guess I'd do away with any ext. IF filters. That makes sense! Being able to vary the first LO in large steps would be usable as a band select, right?I'll see if I can find the detector to inject the BFO at, but if not maybe I'll just try putting a length of wire under the pcb, connected to the BFO, hoping to couple in that way.thanks!

author: ChristofferB
author_rank: Frequent Contributor
timestamp: April 27, 2019, 12:16:49 pm
content: Just tried the setup as suggested, adding an AD831 and an 1MHz LO to the input, and a long lenght of wire. Nothing to be heard. I'm afraid the mixer is overdriving the input, maybe I should try and throw in a largeish pi attenuator inbetween.

author: ChristofferB
author_rank: Frequent Contributor
timestamp: April 27, 2019, 07:44:27 pm
content: Okay, see below for version 1. Coupling is 6 turns of wire around the ferrite antenna directly to the SMA connector on the chassis. Mixer is an AD831, fed RF via ~8m wire, and fed LO via an AD9833 DDS generator.Setting the 1st LO (DDS) to either 1,2,3 or 4 MHz, and tuning with the AM radio tune wheel, I can hear the whistling of running over a carrier signal, and the frequencies changes with different LO freq's, so it's definately not just the ferrite coil picking up signals.I can't hear ANY modulation, though. neither voice, nor digi-modes. No CW either. I haven't done any work on adding a BFO yet, but shouldn't digi-modes be listenable anyway? Can you recognize an SSB station without a BFO or do they just sound like a blank carrier?thanks!

author: @rt
author_rank: Super Contributor
timestamp: April 28, 2019, 09:18:49 am
content: If your intelligence is already on the IF, and you are injecting signal into, or after the mixer, the front end is still mixing whatever tuned signal from the antenna, also appearing on the iF.You could disable the LO and front end in that case to shut it up.

author: vk6zgo
author_rank: Super Contributor
timestamp: April 28, 2019, 11:07:08 am
content: Quote from: @rt on April 28, 2019, 09:18:49 amNo, he is injecting the output of his external mixer into the tuned input circuit of the MF AM radio, giving him a "double conversion" Superhet.OP, you need to have at least a band pass network before your first mixer.Probably the best way would to have a tuneable LC network which you can "peak up" in order to get the strongest signal level.By the way, do you have another source of HF, beside your LO one?If so, you can check that the circuit, even as it stands, works.Another point, is that HF bands are sometimes very lacking in signal if the propagation is not good for your location, so there may be no signals to hear.Edit:- I now see you have a amp before your mixer!The "carriers" you hear may be switchmode power supplies!

author: ChristofferB
author_rank: Frequent Contributor
timestamp: April 28, 2019, 12:20:22 pm
content: Quote from: vk6zgo on April 28, 2019, 11:07:08 amI have a 455 KHz oscillator. I'll look if I have some 1-7 MHz TTL oscillator cans lying around, I guess one of those and a pi attenuator would be a decent LO source.I think the front end is working. I've basically just tapped into a previous DIY receiver that worked quite well.I'm still most concerned that I may be overdriving the ferrite rod LC circuit of the AM radio, since the mixer throws out -30 dBm on the IF side.I'll try putting an LC tuner in front and see if there's any difference. I can usually always hear weather RTTY from Hamburg (DDK2) here.Yeah I have a 30 db LNA before the mixer.Thanks for all the insight! I guess this is slightly bigger a project than I was expecting!--Chris

author: ChristofferB
author_rank: Frequent Contributor
timestamp: April 29, 2019, 12:52:03 am
content: ha! solved it! My RF preamp was dead! With just the mixer connected to a 15 m lenght of antenna wire and a 4 MHz XO can oscillator (with attenuator) as LO I can very clearly hear some of the classic stations in the 4-mhz region: DDK2 (german weather RTTY) and UVB-76 "The buzzer".More experiments to follow! Thanks for all the insight!

author: vk6zgo
author_rank: Super Contributor
timestamp: April 29, 2019, 02:12:44 am
content: Quote from: ChristofferB on April 29, 2019, 12:52:03 amMagic!After my last posting I started to look around for AD831 information.Apart from the PDFs there was little,except for a few YouTube clips which didn't add much.Ididfind a clip by W2AEW, where he used a mixer (a homebrew one) to up convert HF to the frequency range used by the common SDR "dongles"As just a quick demonstration, he used the mixer fed straight off the antenna, just as you now have, with a signal generator supplying a 120MHz LO oscillator signal.(maybe another path for your further experiments).As in your case, the direct connection was quite effective---much more so than I would have anticipated.W2AEW's video clips are really informative, & he doesn't have a hectoring tone.He talks as if you are a visitor to his "ham shack" to whom he is showing some neat tricks.

author: CJay
author_rank: Super Contributor
timestamp: April 29, 2019, 07:47:54 am
content: Quote from: vk6zgo on April 29, 2019, 02:12:44 amSeconded, he's a good guy and he makes an excellent teacher, he turns up here every now and then too.