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How do I build a tuneable band-pass filter ?
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vk6zgo:

--- Quote from: jeremy on May 25, 2012, 10:16:12 pm ---Thanks guys, that Scotty one looks pretty good. I have been looking at the design for the hp 8563e mostly, service manual is here. The functional diagram is on page 347, and on page 351 it has the following:


--- Quote ---A10 YIG-Tuned Filter/Mixer
The YIG-tuned filter/mixer (RYTHM) is a combination of an RF switch, a high band mixer, and a tracking preselector. The PIN diode switch directs the RF input to the appropriate mixer in the A10 RYTHM assembly or A8 low band mixer.

The tracking preselector is a YIG-tuned filter. It functions as a tunable bandpass filter for high band signals. Coarse frequency control originates from slope and offset DACs located on the A14 frequency control assembly. (Slope and offset DAC values are loaded into EEROM.)
Fine frequency control originates from a preselector peak DAC located on the A3 interface assembly. Values for the preselector peak DAC are interpolated approximately every 17 MHz based upon data taken during the frequency response (flatness) adjustment. The preselector's bandwidth varies from greater than 30 MHz, at 2.75 GHz, to greater than 60 MHz, at 26.5 GHz.
--- End quote ---

Why is there a tuneable bandpass in this design? Perhaps I am misunderstanding its purpose.

As an aside, is there actually a way to build a tuneable bandpass over this range ?*

--- End quote ---

The idea of this is to add some selectivity prior to the actual Superhet part of the device.
Most "Homebrew" units forgo this in the interests of simplicity.

*If you are HP/Agilent, have a team of Development Engineers,have been making these things forever,& have an adequate budget--Yes!
jeremy:
Thanks guys, this makes a lot of sense. I've just discovered mini-kits sell mixers for fairly cheap prices so I might grab a few and give it a play.
siliconmix:
 has you looked at any schematics ?
jeremy:
Yes, I have looked at a bunch. They are mostly HP ones though, so they are quite a lot more complex than I need.
kipp:
Hi,

This is an old thread, but I thought it would be worth pointing out that the answer is probably different today than it was in 2012.  Today, direct digital conversion is probably the best idea for a DIY project like this, since you're only interested in frequencies up to 50 MHz.  100 M sample/s is enough to digitize everything from 0 Hz to 50 MHz all in one go, and that kind of ADC chip costs less than $20 in single unit quantities (as of 2023).  You would still need to design input conditioning (attenuator, low pass filter, amplifier) to get the input signal's amplitude into a range suitable for the ADC chip, and a microcontroller with enough speed to capture a run of samples into ram chip, and then shuffle them from there into a PC over a USB connection, but after that the spectrum analyzer becomes a software problem.  Research free open source "SDR" (software defined radio) projects to get ideas from things other people have done.
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