I promised to have a look at the diode detector, particularly what can be expected with a homebrew implementation. I don’t want to hijack this thread, hence would like to keep this post short, but there are several aspects that might be of interest for anyone having an SA, hence dealing with HF. So here it goes:
The diode detector consists of two parts – a dummy load and the detector itself. If the dummy load doesn’t work reasonably well, the detector cannot work satisfactory either. So I wanted to know what can be achieved here with very little effort.
I decided to use two 100 ohms / 2W metal oxide resistors in parallel, so the detector can handle powers up to 4W = +36dBm. No attempts have been made to preserve the characteristic impedance, so I had no high expectations. But surprisingly, the return loss is near perfect up to 770MHz and still decent enough up to some 2.4GHz (RL_MOX_2x100R_3000MHz_2)
Please note that the directional coupler used for this measurement is only specified up to 2GHz and even though I’ve calibrated for an “open” over the full frequency span, the results might not be totally reliable for frequencies >2GHz.
Here is a picture of the test assembly, exemplary for the peak-peak detector (Detector_1N6263_Vpp)
I have tested several configurations:
1: Germanium diode AA116 with 1nF capacitor, just as in the article posted in reply #12.
2. Small Signal Schottky diode 1N6263 with 1nF capacitor.
3. Small Signal Schottky diode 1N6263 with 27nF + 1nF capacitor.
4. Voltage doubler (for peak-peak detection) with 1N6263 and 68nF capacitors.
As a reference, I’ve also evaluated a demodulator probe from TESTEC, the TT-DE 112, which is specified up to 950MHz / 3dB. This is an oscilloscope probe and has no internal dummy load, hence I’ve just used to measure the e.m.f. of the signal generator and consequently the voltage output is 2 * Vpp.
First the transfer curves measured at 10MHz (DD_TF)
The TT-DE 112 looks way more sensitive, but this is because it actually sees twice the voltage. Nevertheless it’s quite impressive, as the error at the specified “turn-on” voltage of 200mV (which corresponds with 100mV in the diagram) is just -23.63%.
All the homebrew detector circuits generally show an error of some -10% even at higher voltage levels. The following diagram shows the error for the various implementations and here the germanium diode certainly looks best (DD_TE)
Now for the frequency response at 1Vrms (2Vrms for the TT-DE 112) up to 1GHz (DD_FE)
As expected, the germanium diode only works up to some 170MHz – it is just not fast enough. Other types might give better results, but then again, the AA116 is intended for detector circuits, albeit for low impedance ones. The high impedance detector AA119 would have been better suited for this application, but I don’t have one here in this lab and it appears to be heavily sought after and not easy to obtain, let alone for a reasonable price.
The 1N6263 is not happy with just 1nF, as it shows a resonance with +8dB at 800MHz. This was the reason why I added a 27nF capacitor right behind the diode and this looked way better, but still climbs up to +7.44dB at 1GHz.
Finally the voltage doubler configuration with two 1N6263 diodes and two 68nF caps, shown in the diagram as “1N6263-3”. The frequency response looks similar to the previous configuration, but it outputs twice the DC voltage.
The TESTEC TT-DE 112 excels here, not exceeding its specified +/-3dB error up to 1GHz.
As a conclusion, the TT-DE 112 together with a good quality pass-through terminator would be the best solution after all. Commercially available pass through terminators are usually rated to 2 watts so it almost doesn’t pay off to build your own using MOX resistors, just to gain a couple of watts. Rather use a power attenuator in front of the pass-through terminator if needed.
Right now I don’t have an accurate RF signal generator for frequencies >1GHz , but expect to get one within the next couple of weeks. Then I should be able to check the behavior of the TT-DE 112 for frequencies up to 2.5GHz.