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. The impedance of the signal path is 50 ohms. I think I have a way to do it using 2N7002's but it requires a lot of parts. Is there a device similar to the 3306 that might be useable for this application?
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.
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John Larkin Highland Technology, Inc
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What is the structure of your front-ends? Some of the old original Motorola "RF" chips simply switched input stages (current-controlled). ...Jim Thompson
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I love to cook with wine. Sometimes I even put it in the food.
The solution used by equipment manufacturers for decades is a bunch of diode switches. 3.5 and 7 MHz are not considered worth RF cmponents in the current regard.
The simplest solution uses one series diode switch at each filter end (4 total), and you can use more for better isolation, e.g. a 3 diode T switch instead of just a single series diode.
For examples, get te ARRL handbook and read the chapters on receiver design.
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Tauno Voipio, OH2UG
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A PIN diode switch is what is usually used. However, for 7MHz and down, you could probably use any diode. There will be some loss due to the on resistance of the diode, but at 7MHz and down, it won't have a big effect on overall performance due to the predominance of atmospheric noise. If my guess(tm) is correct, you'll be switching both ends of your bandpass filters, requiring two SPDT switches.
An SPDT PIN switch will require either 2 or 4 diodes. Two in series with the RF path to switch on the desired filter. Optionally two more to ground, to shunt out the filter that's turned off. Make sure that there's enough forward current through the series diodes so that an incoming signal doesn't turn off the PIN diode for part of the cycle. If your receiver happens to be near a transmitter, this is an important consideration.
The disadvantage of the PIN diode switch is that is sucks power. If you're building a battery powered portable receiver, this can be a problem. The common alternative, as used in the Elecraft K3 and others, are latching relays, which are both low loss and low power consumption.
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Jeff Liebermann jeffl@cruzio.com
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Yep, but for 7MHz, he doesn't need 900 MHz performance. The US3USB30 has an on resistance of 6 ohms typical. The OP posted that he found the TS5A3160 to be adequate, which as an on resistance of 1.1 ohms. For a 7MHz max system, either will work, but the latter is less lossy.
However, I think both chips will be a problem in a high RF environment. Both switches seem to be designed for switching digital logic, not RF. Neither data sheet shows how much voltage swing can be pushed through the multiplexer before it clips. My guess(tm) is about
50% of VCC. Assuming Vcc = 5V, that would 2.5v p-p or 0.88v RMS. In a 50 ohm system, that's: P = E^2/R = 0.88^2/50 = 15 milliwatts = +12 dbm Let's see if that works for a good receiver, such as the Elecraft K3: IM3 (20KHz 14Mhz) = +29dBm. A good rule-de-thumb is that the 1dB gain compression point is 10dB below the third order intercept. That would mean the receiver would need to handle up to an input level of +19dBm. Having the input switch trash the signal at +12dBm will not work.
We can work it backwards. Let's say that the diode switch has to handle +20dB (I like even numbers) or 100 mw. That's 2.23v RMS or
6.3v Peak-to-Peak. This is not going to happen with a 5.0VDC power supply on the diode switch. Even a PIN diode switch is going to need more than 6.3V DC reverse bias to make sure it doesn't conduct from the high level receive signal.
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Jeff Liebermann jeffl@cruzio.com
150 Felker St #D http://www.LearnByDestroying.com
Santa Cruz CA 95060 http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558
I'm using them in a photodiode amplifier, with 3.3 volts Vcc. I have three gain stages, and use one of these to switch the gain of each stage, so I wind up with
2:1 gain steps from 1 to 32. It works rail-to-rail, in fact about 0.6 volts beyond the rails.
It's really just a CMOS analog mux, but it's priced like a USB part. A diode switch, PIN or not, would be much more complex, need a bunch of drivers and inductors, and wouldn't work down to DC.
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John Larkin Highland Technology Inc
www.highlandtechnology.com jlarkin at highlandtechnology dot com
Precision electronic instrumentation
Picosecond-resolution Digital Delay and Pulse generators
Custom timing and laser controllers
Photonics and fiberoptic TTL data links
VME analog, thermocouple, LVDT, synchro, tachometer
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Even 0.6v above the 5V rail is going to cause clipping if the receiver requires a minimum of 6.3V p-p swing. See my (backwards) calculations for handling +20dBm receiver input power. I'm not saying that an analog switch won't work. I'm saying that running from 5VDC, it will not deliver an HF receiver with decent overload specifications. Even if it could go rail to rail, connecting the receiver input to +5V rail on the positive signal input swing, is guaranteed to couple power supply junk into the antenna input (unless the 5V line is VERY clean).
Personally, I prefer a latching relay. Nothing to go non-linear or overload. No power dissipation except when switching. Near zero turn on resistance. No added noise. Bandwidth limited by construction, not components. Easily expandable for more filter sections. Etc.
I agree that a diode switch would be more complicated, occupy more real estate, probably cost more, requires a fairly high turn off voltage, and require a fairly high turn on current. Since there's current going through the junction, there's also some Johnson shot noise added. Certainly not the best technology available. However, with all the RF junk that comes down from an HF antenna, which this switch would be expected to survive, I would not use anything that was ESD sensitive, low voltage, subject to overload problems, can't handle many joules, or is difficult to replace. Induction from a nearby lightning hit, coupled to the antenna, would probably destroy the difficult to replace CMOS switch, while the more sturdy PIN diode and latching relay would be more likely to survive.
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Jeff Liebermann jeffl@cruzio.com
150 Felker St #D http://www.LearnByDestroying.com
Santa Cruz CA 95060 http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558
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