Will this transceiver schematic by VK3AJG reject the other sideband?
Will this transceiver schematic by VK3AJG reject the other sideband or not?
http://members.ozemail.com.au/~jgpri...ansceiver.html
Possibly by filters, not quadrature modulator/demodulator, as you can see from the schematic.
Yes, this is by the filtering method, but I was not sure if the crystal filter, the way used, would be able to reject the opposite sideband.
I agree, the crystal filter bandwidth should be narrow enough to allow only one sideband through but it can't reject the other sideband as such. Which sideband gets through the filter depends on which is aligned with it by the receiver tuning.
I'm concerned about using an LED in the transmitter antenna feed, especially after the LPF. I'm not sure about 1N4007s as RF switches either, I'm sure more appropriate diodes could be used.
Brian.
The LED is used to indicate transmitting, so it could be omitted. How about 1n4148 instead of the 1n4007?
The problem with any LED in the RF path to the antenna is it will distort the signal and create additional harmonics. I would leave it out altogether.
The purpose of D1 and D10 is to isolate the transmitter and receiver from the antenna. When transmitting the receiver should be blocked as much as possible to prevent overload and when recieving, the transmitter should be isolated so it doesn't attenuate the signal. In an ideal World, the diodes would be perfect switches but of course that never happens. The best diode for the job would have a very low junction capacitance when not conducting and a very low forward resistance when passing current. The type of diode with this characteristic is called a PIN diode and I would suggest type HSMP-3810 would be suitable in your schematic. All diodes will work to some extent but PIN will give superior results. I'm suprised at seeing a 1N4007 being used because they have extremely poor RF characteristics and high junction capacitance.
Brian.
Regarding 1N4007 as RF Switches. I use them in professional applications at 13.56 MHz. A saw them in several other commercial equipment also. They are cheap and easy to get.
At moderate DC current (say about 20.. 50mA), you can switch 1A RF. In anti-parallel operation, you can switch several ampere with relative low harmonic distortion. The trick is in the long reverse recovery time (many us). When reversed biased the capacitance is in the 4..10 pF range, low enough for many applications.
At 3.6 MHz harmonic distortion will be higher (as same current at low frequency requires more charge exchange), but given the low power power levels in this TRX, 1N4007 is OK.
I doubt whether HSMP 3810 will be better in this application (given the price), as this diode is relatively fast (when having 3.6 MHz in mind).
I stand corrected, thank you WimRFP.
It is many years since I used anything but 'real' PIN diodes for this kind of switching but most times I'm working at 400MHz+ where the capacitance is far more critical. I have noticed even at low frequecies, the IN400x series vary greatly from one manufacturer to another so it may be worthwhile experimenting with different makes to see which performs best.
The comment about the LED in the antenna circuit still stands...
Brian.
@betwixt
Regarding the LED in the RF output: I fully agree with you.
As the LED works as rectifier also (otherwise it doesn't produce light I think), it behaves as a non-linear part producing harmonics. I also would remove it, or modify the transformer so that it produces just enough light to be visible. Over here, requirements for home built amateur equipment are less stringent then for professional equipment. for 1W output, required harmonic suppression is 43 dB.