How do I build a tuneable band-pass filter ? - Page 1
Forum section: electronics / general

author: jeremy
author_rank: Super Contributor
timestamp: May 25, 2012, 01:24:50 pm
content: Hey all,As of late, I have really wanted to get into RF. I have my microcontroller stuff down, but I really don't like that most of this RF stuff seems like magic to me. Thus, my new project is to build a lowish frequency spectrum analyser (say 500kHz - 50MHz) and I have been reading up on how spectrum analysers work. I am a little stuck on a few things so I am hoping I could get some advice from the RF magicians who visit this forum.I have been looking through old service manuals and it is clear that YIGs are the way it is normally done. I don't want to use a YIG filter/oscillator for three reasons:they are expensivelow frequency ones are hard to findI don't want to worry about about high frequency microwave coupling and noise; YIGs are intended for multi-gigahertz filters/oscillatorsI have a few ideas how to build a VCO that can do what I want (although suggestions would be more than welcome!), but the thing I really have no idea about is how to build a electronically tuneable bandpass filter that can work over such a large range. If anyone could give me any ideas of things to research and/or proven designs I would greatly appreciate it!One last thing to keep in mind: I am still a university student so I don't have a great deal of money to spare. But on the upside, I may be able to sneak into a lab and spend a few quality minutes with a VNA if I can build something to test!

author: joelby
author_rank: Frequent Contributor
timestamp: May 25, 2012, 02:34:39 pm
content: Spectrum analysers are usually superheterodyne radios with fixed filters. Different bandwidths are achieved by switching between different band pass filters, often with relays. A YIG oscillator is essentially a VCO with a very wide tuning range, not a filter.There are a few spectrum analyser projects floating around that might give you some ideas. The most comprehensive is probably theScotty Spectrum Analyzer. Here's amuch simplerone that has a nice block diagram and explanation.

author: ejeffrey
author_rank: Super Contributor
timestamp: May 25, 2012, 02:36:19 pm
content: What kind of design are you looking at? If you build a standard double-conversion spectrum analyzer you shouldn't need a tunable filter. Your signal chain looks like:Low-pass 50 MHz -> mixer -> high IF filter -> mixer -> low IF filter -> detector -> video filterThe first mixer is where your swept VCO goes, and upconverts your signal to an IF that is above the maximum input frequency. This provides the image frequency rejection. The second mixer uses a fixed frequency LO to downconvert to the low IF for your RBW filter, then comes your power detector and video filter.Search google for scotty spectrum analyzer which is a home-brew 1 GHz spectrum analyzer.

author: jeremy
author_rank: Super Contributor
timestamp: May 25, 2012, 10:16:12 pm
content: Thanks guys, that Scotty one looks pretty good. I have been looking at the design for the hp 8563e mostly, service manual ishere. The functional diagram is on page 347, and on page 351 it has the following:QuoteWhy 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 ?

author: amspire
author_rank: Super Contributor
timestamp: May 26, 2012, 12:21:47 am
content: What you have is a fixed bandpass filter. Because if is fixed, you can design and tune it to have exactly the shape you want - something that is extremely hard to do with tuneable filters.You then mix the incoming signal with a low distortion variable frequency sinewave to transform the frequency of the incoming signal to the filter's frequency. Often two conversions are needed if you want to tune from low frequencies to high frequencies. If you were just looking at one band, you could get away with one mixer.In terms of doing it cheaply, the cheapest way I can think of is to get a TV tuner module that will probably be able to tune from less than 100MHz to over 800MHz. You mix the incoming signal with a fixed frequency sinewave at, say, 700MHz and then use the tuner to tune between 700 and 800MHz to get a near DC to 100MHz output. There are sources of affordable mixing modules such Mini-Circuits:http://www.minicircuits.com/products/Mixers_what_is_new.shtmlThe TV tuner will have an output bandwidth of probably a few MHz, so add an extra sharp tuner on the output. An AM band IF tuner would reduce your bandwidth to something like 20KHz.You can use junk parts from TV's and VCR's but finding specs can be hard. There are a number of projects using selected tuners on the internet, or you can just chase after new tuner modules that do have good specs.Richard.

author: vk6zgo
author_rank: Super Contributor
timestamp: May 27, 2012, 04:55:40 am
content: Quote from: jeremy on May 25, 2012, 10:16:12 pmThe 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!

author: jeremy
author_rank: Super Contributor
timestamp: May 27, 2012, 06:42:31 am
content: 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.

author: siliconmix
author_rank: Frequent Contributor
timestamp: May 27, 2012, 09:11:19 am
content: has you looked at any schematics ?

author: jeremy
author_rank: Super Contributor
timestamp: May 27, 2012, 09:12:22 am
content: Yes, I have looked at a bunch. They are mostly HP ones though, so they are quite a lot more complex than I need.

author: kipp
author_rank: Contributor
timestamp: June 14, 2023, 08:47:44 am
content: 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.