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A simple superhet - The RadioBoard Forums
Hi Coildog
... tuning steps etc. Any birdies? Can you estimate how much you spent? Excluding of cource any 3D printers and stufff like that!
How much I spent: A lot. This simple receiver came at the tail end of another project to construct a very simple single band SSB transceiver, and that was a lengthy ( three years ) and somewhat expensive project because I took the opportunity to experiment, a lot, over the course of the exercise with many different parts and circuits. As many as I could get my hands on, actually. I had to upgrade my test equipment along the way. I got one of the new 8 bit digital storage oscilloscopes and a 60 MHz function generator, a whole bunch of filters, parts, pre made modules, etc; along the way it became clear it was finally time to switch from leaded parts to SMD so that meant buying a bunch more parts, and, finally, I learned / concluded the FFT function of the 8 bit oscilloscope was inadequate to really see what was going on and was likely actually misleading me, and would never be adequate to verify compliance with the FCC regulations on spurious emissions, so I bought a 14 bit Red Pitaya ( best piece of test stuff I have ever bought ) to finish that project. Every RF hobbyist should get a Red Pitaya.
So if one already has adequate test equipment, I would estimate the basic electronic parts cost of a simple receiver like this would probably be around $125 - 150 USD exclusive of knobs, chassis, etc. The most expensive bit is the QRP Labs synthesizer at $33 USD. The little SMD breadboards are about $4 USD each. The Mini Circuits RLP-30 is about $15 USD. One could make their own low pass filter, of course. The TUF-3 first mixer I bought as cheap surplus, but if LW reception is not needed then an ADE-1 would work fine at about $6 USD. The price of 45 MHz filters is all over the place depending on the poles, bandwidth, etc., likewise for the second IF filter. I used a TDA1072 because I had some, a TCA440 would likely work as well but they are not very common here in the states. There might be others. TDA7052 are about a dollar here in the states.
Tuning steps: selectable 1 Hz, 10 Hz, 100 Hz, 1 KHz, 10 KHz, 100 KHz, 1 MHz, 10 MHz, 100 MHz. I usually set it to tune in 10 KHz steps, and select 1 KHz if I need to get a frequency ending in 5.
Birdies: I expect it has quite a few as both of the local oscillators come from an Si5351 frequency synthesizer chip, and there is significant cross talk between the two oscillator channels in the two Si5351 VFO kits that I have experimented with. When the radio is in the MW and LW broadcast band both of the oscillators are pretty close in frequency, and a significant amount of the wrong LO frequency is always going to each mixer regardless of the tuned frequency. That said, I haven't really noticed any birdies in the frequency bands I routinely listen to - if they are there, they are below the background noise level. At my location, the RF background noise level can vary from dead quiet to just awful depending on frequency, time, etc. To find all of the birdies would be a pretty big task - I think one would have to select a tuning step, and then tune the entire spectrum from 150 KHz or so to 30000 KHz and log them all. And then repeat for each tuning step. And with and without an antenna to see which ones actually make any difference.
Overall, I am not much of a fan of the Si5351. BUT the Si5351 can generate frequencies well past 200 MHz, so it is really the only easy, cheap, and simple game in town for up converters that use a low VHF first IF that I am aware of. The inexpensive DDS VFO's on the market that use the Analog Devices DDS chip have a lot cleaner (sine wave) output, but these are one channel only and top out at less than 40 MHz or so, so useless for low VHF IF up convertors. There are some problems with these as well, but are easy field modifiable to work right ... . I wound up using one as the frequency control element in my SSB transceiver so am pretty familiar with these as well.
The current broadcast configuration is just a placeholder to have something to listen to while it gets built out at some point in the future to a communications receiver - hence the reason for the SMA connector at the output of the second IF - so I can easily hook stuff up to it for further experiments. The untuned amp means any IF is usable. I have some SMD crystals for conversion from 45 MHz to 455 KHz but had no wide bandwidth filters for that frequency so I got boxed in to using the second channel of the Si5351 for now. Eventually the second IF will be 10.7 MHz, crystal controlled oscillator. I have some really good KVG filters waiting, but there are no all in one chips for that higher frequency AFAIK and I just needed a break for a while ...
... Do you have a wiring diagram? ...
Hi Robert44,
No, I do not have a wiring diagram per se. I tend to think of stuff as blocks, so I have some notes for some of the blocks, but I am unable to attach as the forum software rejects the file size as either a jpeg or pdf. If I get time when I am back at the office, I'll try to rescan the pages from my notebook. All of the circuits in this device are pretty much straight off the data sheet for the part, or are simple generic circuits that are well documented in the usual ham literature strung together to make a functional device. No new ground is broken here!
Be aware that with PIN diodes, frequency makes a difference because of carrier lifetime. The lower the frequency, the more this needs to be considered.
Matching or coupling in and out of the various blocks, and drive levels, that make up anything is pretty specific to the actual parts and frequencies involved that can, and probably would, vary by builder according to the parts he / she can get or has on hand, so not a lot can be gained by specifying parts values for those.
A multiple conversion anything is probably not a good beginner project ( unless it is a well designed kit ) in my humble opinion. Conceptually, they are simple enough, but in practice there is a lot going on with all of the mixing products that just complicates things pretty fast. I learned this the hard way with my transceiver project - I slunk with my tail between my legs in temporary retreat back to single conversion which yielded a pretty decent, legal with margin to spare, QRP SSB rig.
73,
Win W5JAG
