Switching amplifiers, band pass and band cut filters with diodes

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Thanks !

Post by pe2bz on Wed Nov 13, 2013 7:15 pm

Hi Ivan,

thanks for your reply. The good news is that indeed, with only one diode pair the bypass does work now.

Your advise about the DC path to ground took me back to the good old school days. (more than 25 years ago). A resistor to ground gives RF loss. It does work with a resistor for the DC path but I did notice loss of signal. Then I did remeber the use of coils for DC forwarding and RF blocking purpose. I replaced 2 resistors bij 2 coils with a value of 1 uH and that improved a lot. Xl=2*pi*F*L did the rest.

For FM radio band at 87 MHz it's about 550 Ohm and at 550 MHz it's 3400 Ohm so that does not to much load for my signal.

I did buy SMD antenna connector about 4 x 4 mm and patch cables but the connector on the patchcable short circuits my input because it makes connection with the pin diode which is mounted close to the SMD antenna connector. Another lesson learned Embarassed 

I am going to rebuild the circuit, using the patchcables without connectors, soldering them to the groundplane on the PCB and connecting them to the pin diode. I have received 6,8 uH SMD coils so that's even better than 1 uH. Time to heat up the solder iron !

Seems to work quite well.... With no device between my input and output for my "addon devices" I get an isolation of about 40 dB @ 90 MHz and around 30 dB @ 500 MHz. Perhaps some shielding between the input and output would give some improvement but for now I am going to build the second section, connect a Helical 434 MHz filter at the first section and an pre-amp at the second and see if it works as I expect.

Keep you informed,


Kind regards,

Ben

pe2bz

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Re: Switching amplifiers, band pass and band cut filters with diodes

Post by Ivan on Mon Nov 11, 2013 7:19 am

Hi Ben,
- only one diode pair in each bypass will do. The way you designed it, the "inner" two diodes never open in the bypass;
- all the functional blocks must provide DC path to GND both on the input and output, otherwise half of the diodes in the "working" path never open;
- blocking PIN diodes between the signal path and GND, powered in the opposite manner to  those in the signal path, may be required for better insulation.

BR from Ivan

Ivan

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Switching amplifiers, band pass and band cut filters with diodes

Post by pe2bz on Sat Nov 09, 2013 2:10 pm

Hi All,
Currently I am playing around with cheap SDR receivers like the R820T rtl-sdr and with the FCD Pro +

I noticed that the R820T can be used for a lot of stuff depending on what antenna is connected. For reception of amateur satellites it would be great to have a 434 MHz bandpass filter followed by an RF amplifier perhaps with an FM trap in front of it. For DX FM reception however the FM trap should be removerd and also the 434 MHz bandpassfilter would not be needed. However, plugging in and out all these devices depending on what I want to receive is not the best solution so I would like to controll it remotely (at the moment with manual switches) and be able to individually turn on or off one of the devices (the fourt device is a pager notch filter)

What I want to build is a PCB with all the parts I want to be able to control and use pin diode switching to use or bypass each of the 4 items. I allready have RF chokes, BAR64-06 dual diodes and SMD relays and I did make a diagram where I think with all switches in the "open" position the signal goes nicely from antenna input to "out to RX" without filtering or amplifying and with either one or more of the switched closed this diasbles my "bypass" and enables the choosen filter or amplifier.

I did try to ask on a dutch forum if this would work or I could improve anything but the reply stated that I was looking way to difficult solutions and a diagram of a diode switch with four inputs and one output was displayed as my solution. However, I don't want to choose for either one of the four filters, I want to be able to place them in various combinations between my antenna and my RX. I also have SMD connectors and smd to F connector patch cables so I can design my filters in a metal case with 2 F-type connectors and have my own multifunctional switch.

Can someone take a look at my design and tell me if this could be a working solution and if it's possible / necessary to simplify the diagram ?

If I did it right, the diagram should be http://i56.servimg.com/u/f56/18/60/68/65/4switc11.png

Thanks in advance,

Ben - PE2BZ

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Re: Switching amplifiers, band pass and band cut filters with diodes

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