EMRFD Message Archive 11820
Message Date From Subject 11820 2015-10-21 22:14:19 Neil Martinsen-Bu... LPF vs BPF for homebrew transceiver design
I think that this is one of the best email lists for answers to radio design questions. I hope this question is appropriate here.
I am "noodling" (thanks N6QW) a CW transceiver design that uses an Si5351 along with a single signal direct conversion receiver and a simple transmitter (like this one http://ak2b.blogspot.com/2015/02/i-always-wanted-to-try-making-making-cw.html) all under microprocessor control. My question has to do with filtering at the antenna port.
I know (as AK2B also notes) that the output of the transmitter will need a band-specific low pass filter because of the high harmonic content of the square wave RF generator. On the other hand, the DC receivers in EMRFD generally use some kind of bandpass filter on the antenna port. What are the trade-offs in using just a LPF for both TX and RX, or alternatively just using a BPF for both?
Does it take too many components to make a BPF that has enough rejection at the second or third harmonic? What impact do the low frequency signals that come through a LPF have on a DC receiver (either QSD or dual-DBM)? If the problem is broadcast-band bleed through, would a single bandstop filter be sufficient?
My vision is for a multiband CW rig with the only band-specific components being pluggable filters at the antenna. I am wondering if they should be either LPFs or BPFs while maintaining spectral purity and an effective receiver.
Perhaps I am too much of a noodler and I just need to try these ideas out, but I appreciate any experience and understanding that you can share.
11821 2015-10-21 23:46:50 Ashhar Farhan Re: LPF vs BPF for homebrew transceiver design Neil,the first choice would be between a low-pass filter or a band pass filter. I suppose that you have already cast your lot with a band pass filter. Now that you are taking up a band pass filter, i would highly recommend a triple tuned circuit. there are creatures all over the HF. The effect of a nice triple tuned circuit is to keep all the shady and rotten creatures out of your front-end.on the HF bands up to 14 MHz, a triple tuned circuit can directly be used to drive a passive mixer as you are planning. here is what you can do :1. fire up the TTC.EXE that comes along with the EMRFD CD, punch in the values and save the circuit.2. fire up the GPLA.EXE (also on the EMRFD CD), and load up the just saved file. play with the Q, tweak the cap values, have hours of fun.When u are done playing, don't forget to build the tuned circuit!Given that you have a signal source in the VFO, you can simply feed it to the tuned circuit you just built as a signal source and tweak it up.Remember, the band-pass filters only show their magic when both sides are at 50 ohms impedance! That means, a 50 ohms resistor across the jack of your oscilloscope or RF probe and another in series with the VFO output.- f 11822 2015-10-22 08:12:53 Tayloe, Dan (Noki... Re: LPF vs BPF for homebrew transceiver design
I would think using a BPF on the TX would create too much loss unless the BPF components are allowed to get “big” (i.e., low loss).
I use a normal low pass filter on the TX, and then place a capacitor across one of the inductors to cause it to resonate (and thus block) at the second harmonic. When doing this, the size of the inductor needs to be adjusted so that the parallel L/C combination produces the desired inductance at the 1x frequency. This results in using an inductor a bit smaller.
Looking at the Excel spreadsheet that I use to do this (why do it by hand every time?), the new L is 0.75*(old L). Pretty simple. I.e., if the low pass filter used a series 0.48 uH inductor, the new inductor would be 0.36 uH. The resonating C across this inductor is picked to resonate with this new inductance at the second harmonic. I calculate this as:
C (in pF) = 1000000/(2*PI()*2*(1xfreq in MHz)*(new inductance))
It is easy to do in a spreadsheet. This provides additional attenuation for the second harmonic. I often see the second harmonic go from -43 dB (FCC specification marginal) to -55 dB down (well within spec) with this simple tweak. I really have not see a problem with the third harmonic using a normal 5 element TX LPF (3Cs, 2 Ls).
- Dan, N7VE
11823 2015-10-22 08:17:56 Tayloe, Dan (Noki... Re: LPF vs BPF for homebrew transceiver design
Sorry, the formula below uses the inductance in uH. The formula is corrected below.