EMRFD Message Archive 14828
Message Date From Subject 14828 2018-05-21 19:25:10 richj_focus mixer if port termination So,I see for DC receivers the diode mixer is terminated with 50 ohms via a diplexer that is also terminated into a 50 ohm common base transistor amp.So why not design the diplexer to terminate into higher impedance so more kinds of amplifier can be used? That is other low noise amp configureations that have higher input impedance.What do you think?Richard 14829 2018-05-21 19:33:14 Ashhar Farhan Re: mixer if port termination Richard,The diode mixer before the diplexer needs to see 50 ohms impedance for best performance. That is the design. You could alternatively build a diode mixer using some transformer trickery (read some cool stuff by chris trask about building these) so that the mixer charactersitic impedance is, let's say 200 ohms. You could, then, use a diplexer and an audio amp for 200 ohms.The diplexer impedance has to be the same impedance as diode mixer. It is not only useless but even performance degrader otherwise.- f 14830 2018-05-21 21:16:01 Tayloe, Dan (Noki... Re: mixer if port termination
For a DC receiver, a post mixer diplexer seems unnecessarily complicated when other simpler (higher performance?) options exist.
A different (simpler) approach is to terminate the mixer in a capacitor (single ended) or two (double ended) such that the system series R and the detector output C-to-ground form a low pass filter. Think in terms of a 0.47 uF Mylar or ceramic type X7R detection capacitor. In this configuration the impedance looks like an open at DC and a low impedance (short) way off frequency. In this configuration there is no “sum” frequency at all, all the power has been converted into a difference frequency. The detection capacitor turns this into an integrating type detector (Tayloe detector) that has no need to terminate the sum frequency because there is none.
It does not hurt performance. In fact the performance tends to improve. Sensitivity increases (due to lower conversion loss) and additional out-of-band signal rejection is achieved via 1 pole of free R/C low pass filtering. One pole is not great, but it can be helpful to knock down large signals 10 KHz+ away depending on the R/C frequency cut off selected. Since the RF input is often not 50 ohms, it is good to under size the detection caps so that the bandwidth is not too tight if the input impedance happens to be quite a bit higher.
Doubly terminated diode mixers work well with the differential input of an op amp, even if one side looks like an open (+ input) and the other side looks like a short (- input). The inverting input uses the system R as the input gain setting resistor.
If you use an op-amp for the first pre-amplifier, op-amps tend to oscillate given an large input “C”. A capacitor across the feedback resistor can kill this while providing gain roll off at unwanted higher frequencies. Something like a 2200 pF across a 2.2K feedback resistor (33 dB gain given a 50 ohm input).
Depending on how the op-amp is biased, the outputs of the detection capacitors may need to be AC coupled into the inputs of the op-amp. I would think this would be needed any time a diode mixer is connected to an op-amp unless you happen to have a split voltage supply (like +/- 12v).
For a 50 ohm system (single ended) or 100 ohm (double ended differential), the detection “C” can get kind of large, on the order of 1 uF. However, even a commonly available 0.1 uF or 0.22 uF detection capacitor buys some protection.
- Dan, N7VE
14831 2018-05-21 21:26:11 Ashhar Farhan Re: mixer if port termination Dan,The question, I think, was directed towards diode mixers. They are far more sensitive to reflection that passive FET switching mixers. What you are describing too is a diplexer, though very functional while being trivially simple to implement. The sum frequency will still need to see proper termination, won't it? I am not sure I have fullly understood where that energy goes.- f 14832 2018-05-22 06:52:32 Tayloe, Dan (Noki... Re: mixer if port termination I have not tried it, but mathematically a diode mixer is similar to a MOSFET type switching mixer in that both types turn on and off and both produce sums and differences. Adding the large C as a detector cap changes the mode of operation. The detector cap is taking an RF pulse from the mixer turning on for half a cycle and produces an average detection voltage for that half cycle. The detector cap is being charged via the series resistance of the system impedance, so a frequency that is far away suffers an R/C roll off.
If the mixer is not driven by a square wave, but rather a sine wave, the mixer will turn on more slowly, so the capacitor is averaging over less than the full half wave. That should drive the peak detected voltage a bit higher.
All of the energy in each RF pulse is used to drive the charge (i.e. the detected voltage) on the detection cap. There is no wasted "sum" product in this mode of operation. The detector tends to "short out" incoming RF signals that are way off frequency, reducing the unwanted junk out of the detector and perhaps helps the mixer to be less sensitive to the impact of other signals on its operation.
This effect out to be pretty easy to see using a signal generator with enough output, mixer, and a scope. It is neat to watch what happens both at the operating frequency and as the input is tuned away from the operating frequency. You can definitely see the R/C roll off. You also ought to see a higher output for this mode vs. using a more complex duplexer arrangement.
- Dan, N7VE
14833 2018-05-22 07:21:51 richj_focus Re: mixer if port termination Well i was thinking that the diplexer would have an input impedance of 50 ohms to satisfy the mixer, and them the diplexer output impedance to be higher to match a non-50 ohm audio amp.Richard 14834 2018-05-22 07:29:20 Ashhar Farhan Re: mixer if port termination Dan,The diode is a non-linear device. Unlike the FET where the channel is largely a linearly resistive path. I say 'largely' only in comparision with the diode. The diode is switched on due to the local oscillator current flowing through it along with the signal current. This complicates the switching when the signal current starts growing large enough to compete with the oscillator current, leading to non-linearity. Passive FET mixers like those in mixers that are popularly known by your name, have no such problem. Due to the bias requirements, diode mixers report much higher losses than the passive mixers. I have never been able to reduce the losses below 7 db.I am planning to get back to playing with passive fet mixers this week, i will experiment with your suggestion and report the results on the forum.- f 14836 2018-05-22 11:28:52 Tayloe, Dan (Noki... Re: mixer if port termination
It does not matter if the diode is not as good of a switch as a FET, the theory of operation does not change. The diode mixer is delivering RF pulses just like the FET, but maybe not doing as efficient job at turning on or off as cleanly the FET, at least at HF.
The only real requirement is that the needs RF pulses with a source resistance to work. I don’t see why this might not work better for a DC receiver than the more normal diplex configuration which throws away half the signal power in an unwanted “sum” product.
- Dan, N7VE