STL link RX

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radium98
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Re: STL link RX

Post by radium98 » Sun Dec 23, 2018 5:22 pm

I need a technical ask ,briefly or deeply .
in picture below that is a normal mixer
two inputs one from LO
second from antenna
my question is if LO is 100mhz and the scond input is 50mhz
entering the dual gate mosfet bf961-960 etc... how it is obtained at the output of the IF transformer 10.7mhz pink or orange or green color the frequency of 160.70 or 139.3 mhz .my precize ask is how it is substracted or added
thank you and merry chritmas.

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Albert H
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Re: STL link RX

Post by Albert H » Mon Dec 24, 2018 2:44 am

It works by simple addition or subtraction. If you want to receive 100MHz with a 10.7MHz Intermediate Frequency, you set the LO to either 110.7Mhz or 89.3MHz. The mixer gives the difference between the Local Oscillator and the received frequency. In reality it will give lots of other products too, but these are filtered out by the first 10.7 MHz IF coil, then (usually) by a ceramic filter or two before being fed to the IF chip for lots of amplification, limiting and demodulation.

Similarly, if you have a Band I receiver that's going to be on 60.4MHz, your LO will have to be at 71.1 MHz or at 49.7 MHz (a common radio control crystal frequency). The signal input to the mixer will have to come through a critically tuned filter at the required receive frequency to remove the possibility of "image" reception - picking up unwanted signals on related frequencies.
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radium98
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Re: STL link RX

Post by radium98 » Mon Dec 24, 2018 11:27 am

Thank you Albert H
But i still not understand where is that added or substracted in the transistor itself ? Or in the IF coil.

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Re: STL link RX

Post by sinus trouble » Mon Dec 24, 2018 4:47 pm

Hello Mr Radium :)

The mixing of the two signals is done by the dual gate FET! As Mr Albert explained, The output will contain a signal which is the difference between the two input signals (IF) amongst many others!

The TOKO coil when tuned will filter out most of these unwanted signals and keeps the IF
In theory you can use any frequency as your IF but 10.7Mhz is usually the industry standard

Please watch the following video in FULL! The type of mixer he uses is not important but it shows a great deal about the type of spectrum you would expect
Also a ceramic filter is used at the end of the video to show how it filters the spectrum!

I am as stupid as I look! :|

Albert H
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Re: STL link RX

Post by Albert H » Tue Dec 25, 2018 3:09 am

radium98 wrote: Mon Dec 24, 2018 11:27 am Thank you Albert H
But i still not understand where is that added or subtracted in the transistor itself ? Or in the IF coil.

The mixing that's generating the sum and difference signals actually happens in the FET or transistor. The Intermediate Frequency (industry standard for VHF and UHF receivers is 10.7 MHz) is selected by the first IF transformer - it should be the predominant product. I use three different types of mixer depending on application and frequency of operation:

The dual-gate FET mixer (like your drawing above) is good, and can provide "conversion gain", but is prone to image reception. DGFETs are also susceptible to static, so input stages and mixers can be fragile.

Another approach is to use a single-gate FET, with the incoming signal applied to the gate, and the LO signal fed into the source. This has less sensitivity than the DGFET approach, but is generally more robust.

The last way is to use a bipolar transistor. You need one with lots of gain at your frequency of interest - an Ft of 1GHz or more is best. The incoming signal goes into the base, and the LO into the emitter. This is a fairly "old-fashioned" way to do the mixing, but is very robust. In some cases - especially where there's a lot of transmitter power close to the link receive aerial - the bipolar transistor is the only way to go.
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Re: STL link RX

Post by radium98 » Tue Dec 25, 2018 3:59 pm

thank you Albert H for the explanation and clarification .Also thank you sinus trouble.

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Re: STL link RX

Post by Albert H » Wed Dec 26, 2018 4:08 pm

You're welcome. Happy Christmas!
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