If you are telling Terrell that his approach doesn't work, you are wrong. A 3dB pad attenuates reflected energy by 6dB, and it's a common approach that doesn't require a diplexer. It only works if the SNR effect is allowable. The diplexer works when you need to remove more of the reflections.
op band and transition band by REFLECTING energy back to the input and thus CANNOT present broadband 50 Ohms. It has no other option. Ideal L C comp onents cannot dissipate energy. So to have a wideband 50 Ohm input you NEE D to use a diplexer configuration of HPF with LPF with a dummy load or a pa d.
you must have missed the last 3 words of my post
"or a pad"
m
We call it a "diplexer."
It is standard fare for implementing a wideband load for a mixer.
I've been a little short of time these last few days and I haven't the opportunity to put together something polished. Then again, these little circuits are very simple. See .
I've been using them to hide impedance excursions of matched-input impedance LNAs and to simulate the frequency responses of gadgets that do not fit on my lab bench.
Jeroen Belleman
o
What is the sampling rate? How well will the high frequency stuff go throug h the amp, or will it generate spurious stuff in the amp? While terminating (not reflecting back) all frequencies is a goal for the mixer, some level of rejection of the unwanted "high" frequencies is also implied. What is th e rejection mask?
The "normal-simple" way is to use a straight 3 dB point overlap LPF-HPF dip lexer where the HPF leg dumps into a 50 ohm term. 200 kHz and 200 MHz are 3 orders of magnitude apart, so low order filters (even 1st order?) should s uffice minus additional requirements.
A trouble you may have if the LPF corner is really low, is that the LPF ind uctor (at the diplexer common) won't be an inductor at 1 GHz, and the HPF c ap (at the diplexer common) won't be a cap. Your desired return loss may no t pan out. Of course, a conical inductor is great, but very expensive, frag ile, and Production hates them. There are some tricks, but the higher you c an push your corner-overlap up, then the easier it is to make the high pass leg look like a good term at 1 GHz. In principle, you can use multiple dip lexers. (Split once, split again.) Use "real" parts in your sims.
to
pis
a
ugh the amp, or will it generate spurious stuff in the amp? While terminati ng (not reflecting back) all frequencies is a goal for the mixer, some leve l of rejection of the unwanted "high" frequencies is also implied. What is the rejection mask?
iplexer where the HPF leg dumps into a 50 ohm term. 200 kHz and 200 MHz are 3 orders of magnitude apart, so low order filters (even 1st order?) should suffice minus additional requirements.
nductor (at the diplexer common) won't be an inductor at 1 GHz, and the HPF cap (at the diplexer common) won't be a cap. Your desired return loss may not pan out. Of course, a conical inductor is great, but very expensive, fr agile, and Production hates them. There are some tricks, but the higher you can push your corner-overlap up, then the easier it is to make the high pa ss leg look like a good term at 1 GHz. In principle, you can use multiple d iplexers. (Split once, split again.) Use "real" parts in your sims.
"I cannot know what antenna might be used"
Yeah, RF isolation isn't infinite, there will be harmonic mixing possible, and other indeterminate products sneaking in for an "open to the world ante nna." A filter somewhere prior to the mixer might be nice too. Not sure if it is practical. So you have a single conversion cascade?
All good points. Using two diplexers is pretty easy and a good idea. Especially since I'd like to increase the upper frequency limit, if I can work out how to easily produce two phase-locked signals, f0 and (f0+IF). The first is the analyser stimulus, the second used to demodulate signals from the port couplers, preserving RF phase in the IF phase. I can do that to 200MHz using an AD9959 DDS.
The "normal-simple" 3dB way is what I ended up with, more or less; the following active filter can deal with 2MHz but I don't want it to see
200MHz.The goal is to get out of the RF domain as quickly and accurately as possible, into a low-IF signal processing chain I can accurately digitise and process in software.
Clifford Heath.
Is the antenna directly connected to a passive mixer ? In that case, a
-3 dB post mixer pad will degrade the noise figure even further. With an RF amplifier prior to the mixer, the pad losses can be compensated by the RF amplifier.
When a mixer is driven by a square wave, there will be mixing products with +/-3LO, +/- 5LO etc. An octave filter in front of the mixer will help eliminate spurious responses like 2RF-3LO and so on. Also there are images at mRF+nLO which also needs to be terminated. Thus a 200 MHz receiver might have mixing products over 1 GHz.
If e.g. a 1/4 wave antenna is used, it has also nice resistive impedances at 3RF, 5RF, in which the antenna works as 3/4 resp. 5/4 wave antenna resonances. A LPF above highest needed RF frequency helps a lot.
A 1/4 wave antenna doesn't have any resonances below resonance and at those frequencies the antenna is highly reactive with a resistive component of only a few ohms, so it doesn't match well with a 50 ohm input.
For this reason a LPF filter above wanted frequency is important, but in order to avoid harmonic mixing it is a good idea to filter out frequencies below RF/2, if they are strong for some reason.
I'm not really following this, I thought a Chebychev's were 50ohms in and out. (or what ever impedance you build them for) That said this is the only Diplexer I know, it was called DC to Daylight and was for a direct conversion receiver. (50 ohm) Pay attention to inductor resistance and adjust R accordingly. Mikek
Characteristic impedance is the number that keeps coming up time and again in such circuits, but it takes a very special filter to have it all the time!
In particular, the impedance of such a filter varies up and down around the [geometric] mean, by a ~constant amount across frequencies (how much and how often, depends on the filter type).
Think of the impedance of a transmission line stub, it has peaks and troughs but the mean value is Zo.
Butterworth is interesting for many reasons, not just maximal flatness but also for the sharpest(?) filter with a one-to-one impedance curve (i.e., the choke-input kind, the impedance keeps going up and up into cutoff; the cap-input kind, the impedance keeps going down and down).
Cheby doesn't have this property, and so can't be used to make (perfectly flat) diplexers IIRC (if nothing else, the circuit is necessarily more complex).
Tim
Seems I forgot the link for the DC to Daylight diplexer.
Mikek
For Butterworth filters, it's easy. You'd use a filter for a zero-impedance source and put the dual filter in parallel at its input. ('Dual' means a filter with reciprocal element values in the normalized prototype.)
For 'tame' filters, such as Bessel, Gaussian or linear phase with small ripple, a small adaptation as described here can do the job. For more aggressive filters, Chebychev, Cauer and whatnot, I have no good solution. I worked out a procedure for perfectly matched Bessel filters, but I realized that their practical realization wouldn't be any better than the approximate solution described in my web page.
Component parasitics limit the useful bandwidth over which a diplexer will work. That also goes for your DC-to-daylight diplexer. You can extend the bandwidth of the match with one or more constant-impedance bridged-T low-pass stages.
Jeroen Belleman
Have something to add? Share your thoughts — no account required.
Ask the community — no account required