On Sunday, April 14, 2013 1:01:15 PM UTC-7, garyr wrote:
I'm trying to build a 80/40 meter receiver and I'd like to be able to

switch the RF input between the two bandpass filters. This sort of thing is

usually done with relays but I'd like to avoid that if possible. A

SN74CBT3306 would be ideal but I think the on-resistance would be too high,

thought it's hard to tell from the data sheet. Are there other devices that

would be suitable for this application? What do you think about using

2N7000's for this job?



The input and output capacitors of the double-tuned filters are 2200 & 2700

pF so the stray capacitance of the switching circuitry would not affect the

response very much.



Thanks in advance

There are RF solid state switches designed for 50 ohm systems. Several companies have them: Hittite and Peregrine are a couple. At VHF and above they have very good third order intercept performance, but at least if the signals are fairly large, their intermod performance falls apart at lower frequencies. If you're operating below 0dBm, they may work well for you. They are tiny and easy to use...SPDT in a tiny 6-pin SMT pkg.

In an HF receiver I designed a few years ago, I tested many different RF switches. We needed something that would switch quickly (probably not an issue for you) and have very low distortion. The solid state switches I tried all had too much distortion, and I ended up selecting tiny reed relays (Meder SPST and Sanyu SPDT). They also contribute some distortion, but it was tolerable. They switch remarkably fast! The next best that I tried was Panasonic PhotoMOS "relays". They worked OK at 40M and lower frequencies, but had too much (non-linear) capacitance at higher frequencies. I considered diode switching using PIN diodes, but the net solution that way would have taken too much power (diode DC current), and PIN diodes with long enough storage times were pretty expensive.

You posted that you want to avoid using relays...I can appreciate that, but will still suggest that relays are a good way to switch RF with low distortion. Omron makes a nice DPDT RF relay that's quite small...the G6K-RF parts. Problem is that they are rather expensive. However, the same relay is available in a "non-RF" version, and its data sheet lists very good performance at lower RF frequencies...through 30MHz at least. They are relatively cheap, I think under $5 each in small quantities, and switch cleanly with practically no distortion. (I've tested for distortion with +20dBm input, and see distortion products around -140dBc...pretty much at my limit of ability to easily test. At that level, I'm not sure if the distortion is coming from the DUT or from the test setup.)

Cheers,
Tom