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Single Frequency Superhet for WRMI 5050 KHz - work in progress
In Solid State Radios
Single Frequency Superhet for WRMI 5050 KHz - work in progress
In Solid State Radios
w5jag
May 30, 2024
The RF block is approaching finalization, I think. IF Selectivity is provided by two (2) Murata SFU455B ceramic filters. The data sheet indicates, and testing confirms, that although marked as “455" they are, in fact, 462 KHz +- 2 KHz. In most circumstances, one filter might be completely adequate. Here, I have two in series, which somewhat complicates matters as they must be coupled together with a coupling capacitor - which presents some of the same problems as a top coupled double bandpass filter, and the center frequency of each filter may, or may not, be identical. More likely “may not”. After spending the better part of a day playing with these, I settled on a center frequency of 462.250 KHz for the IF, and a 47 pF coupling capacitor. They may still be slightly overcoupled - it’s hard to say with my simple test equipment, and the filters themselves are hardly precision devices. A picture of the resulting bandpass is attached. It's pretty good, I think. dual SFU455B 47 pf coupling I have quite a few ceramic filters, but on the off chance that someone might actually want to duplicate this receiver, I wanted to use parts that are easily available. My first choice was the Murata SFU455A, a true 455 KHz part that I have quite a few of, but for some reason they have become stupid expensive if they were to be purchased right now. The SFU455B looks to be abundant, and, best of all, cheap. I got a bag of twenty (20) on AliExpress for less than $2 USD. The Yellow core IF transformer tunes to 462 KHz easily. 455 KHz IF transformers have become much less available in the last year as Mouser has dropped the common Xicon transformers, but are still out there to be had, and can always be liberated from a junk transistor radio, which might also yield a ceramic filter if it is a better quality radio. The IF amplifier, AM detector, AGC, and audio pre amplifier are handled by a TDA1072 integrated circuit. These IC’s, while obsolete, exhibit no availability issues, are inexpensive, work very well, and require few external components, a quality I think I have previously mentioned that I always find quite appealing. In this application, it looks even more bereft of parts than usual, as I mounted most of the parts underneath the chip using 0805 or 1206 SMD parts. In place of electrolytic capacitors, where audio is not being passed to the listener, I used non polarized ceramic parts to save space. This made it possible to hide large capacitors underneath the chip. All of the caps would be underneath the chip except that I made a mistake with the caps on pin 5, and mistakenly installed 1 nF caps in parallel instead of 100 nF. Instead of removing the chip to fix this mistake, I just paralleled the correct caps to the side of the chip. I’m still in the listening phase right now, but I think things are on the correct track to completion. Sensitivity is good - my house at the lake was in the direct track of the tornados last weekend and my antenna is just gone - as well as a lot of other stuff (like electricity and trees) so I am listening to WRMI with about fifteen feet of wire tossed out on the deck (which is still here, although the heavy outdoor grill is gone pecan - probably landed somewhere in Missouri ... ). The 18650 batteries in the low power AC/DC supply described in another thread are powering the radio. Cell service came back today, before that all I had was ham radio, so at least I now have a wi fi hotspot, and limited 120 VAC from a gen set, an essential household appliance in tornado alley. It gets a lot of use. 73, Win W5JAG getting closer
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Single Frequency Superhet for WRMI 5050 KHz - work in progress
In Solid State Radios
w5jag
May 20, 2024
I used the “genuine” HP mixer chip for a few evenings, and really could not tell any difference from the dubious origin chips in this particular application. Which is good, I suppose, since they are plentiful at relatively low prices on eBay and AliExpress. Regardless, I removed the chip, cleaned the board, and installed an MCL ADE-1+ DBM which is a plastic case, very low cost, level 7 mixer from Mini Circuits and available new from Mouser. I use one of these as the second mixer in my broadcast receiver and have had good results with them. They are a good mixer for something that never sees harsh conditions. MCL specs them from +4 dBm to +10 dBm so really all you have to do is hook the LO up to them and you are good to go without worrying too much about the drive level. Before I installed the ADE-1, I tried out an old SBL-1 that was the go-to DBM back in the 80's and 90's in most of the higher performance homebrew amateur receivers. IIRC, Collins even used them in the KWM-380. Anyway, I’ve had a few of these bouncing around, there is no shortage of space on the board (if the receiver stays single conversion) so I installed it, and apparently it had succumbed to the ravages of time. It mixed properly, but it had loss 30 - 40 dB more than it should, so, unusable. I have a few more of them, so I suppose I need to check the others to see if they are any good. I believe MCL still makes the SBL-1, but they are expensive because of the metal can. Because I have moved from conversion gain to conversion loss, I installed a post mixer amplifier based on an MMBT3904 transistor. The transistor is from one of the variety packs from AliExpress where you get a variety of different types for a couple of bucks. I have had generally good results with these types of parts for hobbyist HF use. The post mixer amplifier works well, at least on the kitchen table. At 0.001 volts at the antenna input, the IF output at 455 KHz is -15 dBm. It clips at about +3 dBm. I have attached the spectrum plots and oscillograms of the mixer / post mixer amp at 0.001 volts antenna input. ADE-1 0.001 volt antenna ADE-1 plus post mixer amp 0.001 volt antenna post mixer amp at 0 dBm output I also took some plots of the selectivity at 455 KHz with a 455 KHz IF transformer as the selectivity device, and the IF transformer plus a Murata SFU455A ceramic filter. I’m not sure if these filters are one or two pole, and the bandwidth is supposed to be 10 KHz. That appears to be true based on my simple measurements. As can be seen, the selectivity with just the single IF xfmr is pretty mediocre - it has a reasonable bandpass, but the slopes are very shallow - at - 25dB, the bandwidth is about 380 KHz to 530 Khz. Better than nothing, but only barely. yellow core IF xfmr Just adding the single Murata sharpens the passband up rather dramatically. With the Murata in place, the - 3dB points look to about 10 KHz, and the - 25dB points are 440 KHz and 470 KHz. This is just a quick test jig; more careful layout and matching of the inputs and outputs might improve this, as would multiple filters, as these are very inexpensive. Insertion loss appears to be insignificant. yellow core IF xfmr plus Murata SFU455A I have not yet tried this on the air, and probably will not be able to until next weekend or the weekend after. Win W5JAG
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1.2-volt regen circuit from German experimenter
In Solid State Radios

w5jag

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