EMRFD Message Archive 2247
Message Date From Subject 2247 2008-10-14 21:19:27 rcbuckiii I need L-C filter suggestions I am putting together an upconverting receiver. I am going to have
3.5-4.0, 7.0-7.5, and 14.0 to 14.5 bands. I would like to use 45 MHz
for the upconversion IF with a 500 KHz bandwidth. I will be using
fixed frequencies for the 1st local oscillator which is why the wide
bandwidth is required. I will subtract the desired frequency from the
local oscillator. And I want to reject the LO+desired frequecy band.
I want to place the bandpass filter right after the 1st mixer.
Using AADE and Elsie gives strange values for the inductor and
capacitor values, such as .5pf and 1.4 nH. Analyzing the filters with
these values gives good results. However, if I try to change the
values to something reasonable, the simulation shows very poor side
slopes. Only 3 dBm difference between the 45 MHz CF and 49 MHz image
for the 80 meter frequencies.
If I change the upconversion IF to 75 MHz, I am able to come up with
reasonable component values and a good response shape. However, that
makes my 2nd coversion down to 9 MHz more difficult.
Does anyone have any suggestions for a 45 MHz bandpass design using
reasonable component values? And it would also be desirable not to
have to go to a 12 order design. Or would it be better to try and
combine a low pass filter with a high pass one to get the desired
2251 2008-10-15 07:20:55 timshoppa Re: I need L-C filter suggestions 2252 2008-10-15 07:47:51 sm5glc Re: I need L-C filter suggestions What kind of specs are you shooting for on 45 MHz??
Just tested a quick 2 and 3 pole filter with the requested 1% BW.
Using Qu of 200 on inductors and 500
2253 2008-10-15 12:24:06 Wes Hayward Re: I need L-C filter suggestions Ray,
There are some third order bandpass filters at 40 and at 50 MHz on
pages 6.85 and 6.88 of EMRFD. These were designed from first
principals, but then run through TTC.exe that is on the EMRFD CD.
The approach taken within that program is to tread the inductors as
the beginning variable and to then calculate the capacitors needed to
obtain a viable filter response. As with any filters, the results
you get are vitally dependent up
2254 2008-10-15 18:50:38 wimmie262000 Re: I need L-C filter suggestions Have you looked in EMRFD at page 3.14? The table seems to give
acceptable values up to 50MHz.
For an accurate value of the small coupling capacitor you could use
the "trick" at the top of page 3.15. (or figure 3.24, but there the
values are not quantified in detail)
2256 2008-10-15 18:50:38 rcbuckiii Re: I need L-C filter suggestions Quote: "I personally don't like the challenges imposed by upconverging
Tim, I'm not looking for excellent performance. Just something for
general listening. My idea was to use a PLL to generate the 3
frequencies for the LO to upconvert to 45 MHz. Then I was going to
mix a ~5MHz L-C VFO with a crystal oscillator for final down
conversion. Just an idea I was playing with. I may end up having to
use the PLL as a tunable 1st oscillator to avoid some of the issues
you mention. That is the way the Yaseau FRG-7 that I have does it.
Lasse, what tool did you use to model your design? I was using Qu of
200 for the inductors. AADE doesn't require Qu for caps. I guess they
assume ideal parts. I'll go back and test the design with 220 nH
inductors. If I can get > 10 dB difference between the desired 41.5
MHz signal and the 48.5 image signal I think that will be sufficient.
2257 2008-10-15 18:50:38 victorkoren Re: I need L-C filter suggestions I don't have any problem using the AADE filter designer to design a 3
pole BPF with practical component values.
A step by step design:
1) file > new
2) design > coupled resonator > butterworth
3) cf=45M bw=600k order=3 >enter
4) Li=0.18u Qu=200 >enter
5) Zin=50 Zout=50 >enter
6) enter a neme for the filter >enter
You get the filter schematic with values.
All the inductors are 180 nHy, resonator capacitors are 62pF and 68pF
and coupling capacitors are 6.4pF and 0.65pF.
All components are practical
Analysis of the filter shows a center frequency attenuation of 6.85dB
Victor - 4Z4ME
2259 2008-10-15 18:50:47 rcbuckiii Re: I need L-C filter suggestions Wes,
Thanks for the information. As usual you come to the rescue. I will
pull the CD out and run those filters through TTC. Then I can make
the adjustments to center one of them on 45 MHz. In the paragraph at
the bottom of 6.84 you say you measured 80-dB rejection of the
unwanted signals. If I can obtain 40-dB I will be happy.
2260 2008-10-15 21:17:55 rcbuckiii Re: I need L-C filter suggestions Victor,
Thanks for the great tip. I was selecting Design -> Butterworth ->
Bandpass. The coupled resonator gives much better results. This is
the same type of filter that Wes was referring to in his DTC and TTC
examples. I had never tried that type of design in AADE. I changed
the two .65pF caps to 1pF and re-analyzed the filter. This creates a
very slight ripple in the passband but not enough to worry about. And
the side slopes are more than ample for my application.
--- In email@example.com, "victorkoren"
> I don't have any problem using the AADE filter designer to design a
> pole BPF with practical component values.68pF
> A step by step design:
> 1) file > new
> 2) design > coupled resonator > butterworth
> 3) cf=45M bw=600k order=3 >enter
> 4) Li=0.18u Qu=200 >enter
> 5) Zin=50 Zout=50 >enter
> 6) enter a neme for the filter >enter
> You get the filter schematic with values.
> All the inductors are 180 nHy, resonator capacitors are 62pF and
> and coupling capacitors are 6.4pF and 0.65pF.6.85dB
> All components are practical
> Analysis of the filter shows a center frequency attenuation of
> Victor - 4Z4ME
2261 2008-10-16 01:29:41 sm5glc Re: I need L-C filter suggestions I just uploaded some screengrabs (in photo folder) that shows how a
3-pole Chebychev filter would look like. Center f=45 MHz, BW=500kHz
and 0.5 dB ripple.
First plot shows an idealistic value of Q>10e5
Second plot shows how the filter turns out with QL=200 and QC=500, not
an impossible feat.
Third plot shows how the fitler looks like with pretty poor Q-values,
QL=125 and QC=200.
What you see is how important it is to have high-Q components when
designing narrow band filters, in this case a BW of ~1%.
If I had to do this, I would look into helical resonators, which
should provide better Q's than those in second plot, but at the cost
BTW, program used is Gensys with filter module by Agilent