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 Post subject: AM radio mod vary bandwidth depending on signal strength
PostPosted: Aug Sun 25, 2019 10:18 pm 
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AM transistor mod to vary the received bandwidth depending on how strong or weak the station is. Why, to get higher fidelity audio on the local stations, but avoiding "monkey chatter" from weak stations' adjacent stations.

Using a JFET as a voltage controlled resistor for variable IF bandwidth, and to act like an AVC circuit in a transistor AM radio. This circuit widens the bandwidth on strong stations (adjacent stations are so weak you wont hear them), for more audio high frequencies, better fidelity. Weak stations will have narrower passbands, avoiding "monkey chatter" from immediately adjacent weak stations.
Image
Here the usual AVC circuit in the AM section of this radio gets an assist from a JFET operating as a voltage controlled resistor. This is in the triode or ohmic region (at and near 0V and 0mA current, the blue region in the figure below) where the FET can operate as a voltage-controlled resistor. The triode or ohmic region in an FET is sometimes known as the linear region. The JFET operating as a voltage-controlled resistor works in this region. If we extend the VDS voltage range to include slightly negative voltages for a particular gate-t-source voltage, we see that the there is still a resistive effect. Preferably, there is no DC voltage across the JFET’s drain and source terminals in the voltage controlled resistance mode. This JFET's source and drain is connected across the entire LC circuit of the first IF transformer, which has no significant DC across it. The source and the drain both have IF frequency energy on them, out of phase with each other. The LC circuit has a tap near the center of the winding, tied to an RF ground (which is why the drain and source has out of phase IF energy) The gate is roughly at the midpoint of the resistance, which should reduce the effect of the gate capacitance on the IF.

When the gate voltage VGS here is dropped below around -4V, the JFET (a J211) becomes near infinite resistance. At around -2.7V, the JFET becomes a resistance around 1500 ohms. This variable resistance across the IF transformer LC circuit attenuates the signal strength, and also as a side benefit, will make that LC circuit have wider bandwidth. Which will allow higher audio frequencies from the strong station to be heard, higher fidelity (also better fidelity because the attenuation keeps the strong station from overdriving and distorting in the rest of the IF strip). Weaker stations will have narrower passbands, avoiding "monkey chatter" from immediately adjacent weak stations.

The controlling voltage is acquired from the AVC line via an inverting transistor (a 2N2222). This transistor level shifts and amplifies the AVC voltage range. Weak signals create a higher AVC voltage, and the transistor goes into more conductance, drawing, via the collector resistor, its collector voltage more negative. Strong signals will produce lower AVC voltages, and the transistor goes nearly into cutoff, and the collector approaches the positive supply rail. Be sure to use a bypass cap between this collector and the B+ line, to avoid ripple and such from "modulating" the voltage controlled resistances of the JFETs. Which would distort the detected audio. You need to size this cap to smooth out ripple, but still allow the circuit to respond to the AVC changing due to tuning in stations. I used a trimpot to get a sweet spot of an emitter resistor value, setting the gain of this transistor (a value that has the JFET do nothing on weak stations, but attenuates strong stations to minimise distortion), then I used a fixed resistor of this value in the finished circuit. Not a great way to do circuit design, as variations in the transistors and JFETs could throw this off in production. The IF transformer's LC circuit is also at the positive supply rail. From the JFET's point of view, (its gate is connected to the transistor collector) its gate going more negative makes the JFET resistance go higher. The gate getting less negative makes it lower resistance, attenuating the strong station's signal. As the AVC circuit is a feedback loop, the JFET's resistance will settle to a value after this loop does its action.

A second JFET on the IF transformer between the first and second IF stages is also installed. You don't want to put it at the last IF transformer, as all you'd do is attenuate distorted signal. A third JFET is also used on the antenna LC circuit (I removed a 1.5pF cap there, as the JFET has some internal capacitance, a few pF's, inside it). Then I retweaked the antenna trimmer cap.
Image
Image
Above shows an exaggerated relation of passbands (how wide some value of X dB down) to radio station signal strngths. The passbands really vary by a factor of two at most. Attenuation is related to AVC action, in that weak stations are amplified more than strong stations in regular AVC circuits, and here there is actual attenuation being done as well (which via the voltage controlled resistive JFETs changes the passband widths of the various IF transformers and the antenna LC circuit).


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 Post subject: Re: AM radio mod vary bandwidth depending on signal strength
PostPosted: Aug Sun 25, 2019 11:21 pm 
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I think I'm following you......do you have it working?

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 Post subject: Re: AM radio mod vary bandwidth depending on signal strength
PostPosted: Aug Sun 25, 2019 11:34 pm 
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Just saw this on Facebook Transistor Radio group earlier...

Very interesting idea and am also asking if you have a working prototype ?

John


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 Post subject: Re: AM radio mod vary bandwidth depending on signal strength
PostPosted: Aug Sun 25, 2019 11:34 pm 
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Which communications receivers had variable IF bandwidth controls?

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 Post subject: Re: AM radio mod vary bandwidth depending on signal strength
PostPosted: Aug Mon 26, 2019 12:17 am 
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I have it working. On two similar GE radios. Found that I did need to filter bypass the gates on the JFETs, or I'd get ripple or such that would contaminate the received strong station's audio.

Schematic of the radio without this mod: http://www.wa2ise.com/radios/t1290sch.jpg
and a web page with the modified schematic http://www.wa2ise.com/radios/amdettransistor.htm#6sb7z (I plagiarized my own writing there to create my first post above) :D


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 Post subject: Re: AM radio mod vary bandwidth depending on signal strength
PostPosted: Aug Mon 26, 2019 9:53 am 
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Quote:
Which communications receivers had variable IF bandwidth controls?


Commonly called "Passband Tuning" - it's on my JRC NRD 515 and I recall it was also on a Yaesu FT 901DM that I had some years ago.

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 Post subject: Re: AM radio mod vary bandwidth depending on signal strength
PostPosted: Aug Mon 26, 2019 2:45 pm 
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At least one modern Sangean radio does this with DSP on the AM band. Now I understand where they might have got the idea, if this sort of circuit has been floating around in the radio hobby experimenter world for a while...

Interesting.


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 Post subject: Re: AM radio mod vary bandwidth depending on signal strength
PostPosted: Aug Wed 28, 2019 9:08 pm 
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Another radio, this one AM, I did this mod on:
Image
Here is a single JFET connected across the 2nd IF transformer T2. The IF amp transistor TR2 also acts like the 2N2222 in the radio in the original post above (as TR2's base gets biased by the AVC line), and the JFET control signal is picked off a bypassed resistor R10, was 1K, now 1.5K (to increase the control voltage range). The full LC circuit of T2 is bypassed by C18 and biased by voltage divider R6 and R7. There was a diode between T2 and R10, but I removed it before I added the JFET. The voltage divider was reworked (by changing R6 to 180 ohms) to have the JFET go almost into infinite resistance when the radio is tuned to an empty spot on the dial. And to drop resistance value on strong signals. This then widens the IF bandwidth and reduces the IF strip's gain.


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 Post subject: Re: AM radio mod vary bandwidth depending on signal strength
PostPosted: Aug Thu 29, 2019 3:07 pm 
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That looks simple enough to be an interesting project !

Could use a pot to control input voltage on gate and experiment with FET's on-hand in parts stash to determine linear region and observe effect on radio performance.

Thanks for posting !

John


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 Post subject: Re: AM radio mod vary bandwidth depending on signal strength
PostPosted: Mar Mon 23, 2020 5:28 am 
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I'm getting ready to experiment with this on a home-made superhet chassis using silicon transistors since the AGC voltage generated at the detector is not sufficient for proper operation. The attenuation will be primarily at the tuner since strong signals badly overload the mixer transistor I'm using.

I have several NOS 2N3955 dual N-channel JFET devices and hope that one of these will do the job---is the type critical ???

Thanks !
John


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 Post subject: Re: AM radio mod vary bandwidth depending on signal strength
PostPosted: Mar Mon 23, 2020 6:58 am 
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But isn't this what AGC does? It reduces the gain of the IF strip by detecting the amplitude of the received signal. If the received signal is weak the IF bandpass characteristic would be a peak of maybe 5kHz wide at the 6dB level, but if the signal was strong the AGC would reduce the gain and so flatten the peak to perhaps 10kHz wide, so giving bettrer fidelity. Or am I over simplifying this???

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 Post subject: Re: AM radio mod vary bandwidth depending on signal strength
PostPosted: Mar Mon 23, 2020 2:23 pm 
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AGC is just a more modern term for AVC and is the functional equivalent. It does not vary the IF passband because only the gain of the stage is changed, not the bandwidth of the filters (transformers).

Also, "Passband Tuning" places a signal at different places within the IF passband but it does not vary the IF bandwidth in the process. It is essentially the same as tuning in a station slightly off frequency to avoid adjacent channel interference. But many receivers, including transistor radios, do have a way to vary the IF bandwidth, but not automatically depending on signal strength.

Dave


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 Post subject: Re: AM radio mod vary bandwidth depending on signal strength
PostPosted: Mar Mon 30, 2020 10:10 pm 
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So you got your GE Superadio with the wide and narrow switch on the front. It seems it would be easier to just convert that switching to an electronic switch that would go from wide to narrow at a predetermined signal level effect based on the AGC. Not the same thing as constantly variable bandwidth but might work OK if you set the the switch point appropriately. IF it were a fading signal it might create sort of an annoying effect if there was a dramatic change in sound between wide and narrow.

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