#400: Measuring the Minimum Discernable Signal and bandwidth of the 40m Direct Conversion receiver.

In today's video, we're going to measure the minimum discernable signal level of my direct conversion 40m solder smoke challenge receiver. The minimum discernable signal level is defined as the RF input level that's needed for the audio output to rise 3dB above the noise floor. And in order to make a consistent point of comparison, this is measured with an audio bandwidth of the receiver at 500 htz. Well, the audio bandwidth of this receiver is not 500 Hz. So, we'll deal with that a little bit later. And in order to make that measurement, you need a meter that can actually read in true RMS, meaning it'll give you a true RMS voltage reading even when the input signal is not a pure sinosoid. And since we're going to be measuring noise plus a sinosoid, we really need that true RMS reading meter. I'm going to be using this old Roden Schwarz SML1 as my RF signal source. Its output can be dialed down as low as -40 dBm, which is well below the noise floor of this receiver. And the meter we'll be using is this vintage HP 3400B uh RMS reading voltmeter. And to actually make the measurement, I've got a 1x probe uh clipped in here to the audio output lead that's going to my external speaker. Uh because of the output stage of this audio amp, the uh output load really needs to be present in order to preserve the frequency response. And because the frequency response rece this receiver is likely not very flat, first thing I'm going to do is just tune the receiver so that I get the maximum output level. So I find the peak of the audio response. So I'll turn the RF signal generator on at a reasonably high signal level. Something would be akin to about an S9 in a receiver. And uh you'll see this meter responds relatively slowly. So I've got to make my adjustments slowly to try to find that peak. Oh, that's going too high and it's going down. Let's go the other way. I'm just looking for a relative peak here. That might be right about there. Let's see if I go a little further. It's coming back down. So, right about there is the peak response through the receiver. I've turned my signal generator now down to minus 140 dBm, which is well below the noise floor of the receiver. And what we'll do is we'll turn the volume up on the receiver and increase the sensitivity of my meter until we get something, you know, it's readable here. It doesn't the absolute reading doesn't matter. We're looking for a relative 3dB change. So, if I go down to the uh 10 molt scale here, I might be able to go even one more down to the 3 m volt scale. And uh let's uh what I'm looking for is I'm going to be reading on this the decibel scale up here. So, uh you can see there's minus5, minus4, minus3, etc. So, if we adjust this carefully, you can see it this receiver is very sensitive to displacement of your hand, and I'm not using really well shielded cables. So, a lot of things are going to affect what we're looking at here. So, uh I'm just going to move my hand away and uh get myself pretty much lined up on, in this case, the minus4 uh dBm signal level. And now I can start increasing the signal level out of my generator until I get a 3dB change. So, I'm bringing my signal level up and I can see that uh there's about a 1 dB change just about a one and uh that there's about two and a half close to three and there's about a 3db change and we can see on the signal generator that's about uh minus 117 dBm which is about 316 nano volts. One more verification that the minus 117 dBm is the minimum discernable signal level. Let's take a look at the meter with the signal generator turned off. And now I'll turn it out output on and look for a 3dB change on that lower decibel scale. And there was our 3dB change. But of course, if we want to compare it to other measurements on other receivers, we need to measure it over a 500 htz receiver bandwidth. And of course, this receivers's bandwidth is not 500 htz. So what we need to do is first measure the bandwidth audio bandwidth of this receiver and then convert down to what the reading would be if we had reduced the bandwidth down to 500 htz. In order to measure the frequency response of the receiver, I'm using this Elecraft uh wideband noise generator uh to the input of my uh receiver. By doing this, I can get a nice uh elevated noise floor so the volume can be high enough to measure pretty easily on the scope. So, I've got

that turned on and input to the receiver. I got them clipping onto that same spot in the receiver to look at the receiver audio. And I've got a uh 4A transform turned on uh to look at the frequency response of the audio output. I've set this to a logarithmic scale looking from about 100 hertz out to a little over 10 kHz. And uh we can see the audio frequency response of the receiver. I'm going to use a set of just X and Y cursors here to kind of mark about where the peak of the audio response is there. And I'll adjust the other cursors here to be about 3dB below that to give me the 3dB bandwidth. So that's uh right about there. And now I'll just move the uh X and Y cursors to those intersection points. Let's move that one here. Let's grab that guy. And I'll move him with the knob over to where it's kind of crossing. Let's say it's probably right about in that area there. And then we'll move the other one up here to where it's crossing to measure the 3dB bandwidth. So that's probably right about there or so. So I'm seeing the audio bandwidth about 3. 3 kHz and that's about what I'm expecting. I'd put a low pass filter uh in the output of the audio amp uh just to limit the high frequency hiss out of the receiver. So a 3. 3 kHz uh bandwidth is about what I expect. Now we measured minus 117 dBm as my minimum discernable signal level but that's now in this 3. 3 kHz uh bandwidth. So 3. 3 kHz is really defining how much noise I'm actually measuring. Again the standard is for a 500 Hz bandwidth. So in order to correct for that we can just look at the ratio of the uh bandwidths and use that to correct our value. So if we take our 3,300 htz measured bandwidth divided by the 500 htz standard bandwidth for making that measurement that's a ratio of uh 6. 6 uh so we'll take uh the log of that and 10 times that and that gives us about 8 dB. So it's about an 8dB correction which means that uh when comparing to other receivers the minimum discernable signal level of this receiver is - 117 minus another 8 dB or about -25 dBm which is about 125 nano volts. Again not too bad for a little five transistor receiver. I hope you enjoyed this video and taking a look at the minimum discernable signal level of my solder smoke challenge direct conversion 40 m receiver and got a peak on how to make those measurements. If you like the video, give me a thumbs up and thanks again as always for watching.