20m receiver, building and repurposing to 27MHz
I am going to replicate the "Simple 20 meter Direct Conversion Receiver" project by MightyDevices (source: http://mightydevices.com/?p=227 ) and I have few questions.
Here is the schematic:
(Source: http://mightydevices.com/wp-content/...4/receiver.png)
Here are my questions:
1. Can I replace MCP6002 with two separate MCP601? MCP601 has Gain Bandwith Product 2.8MHz and the MCP6002 has just 1MHz...
2. This receiver is 20m (14.0MHz), would it be possible to change it to 27 MHz (11 m) aka CB radio band?
3. Can I replace SA612 with NE602? As far as I checked, those are virtually the same chips, but it never hurts to ask...
4. The SA612 here is connected to VDD trough the 100uH inductor. I assume that is somehow needed to filter/stabilize VDD, right? How much his this affecting the reception? Should I keep that inductor in place, or maybe even start adding such "vdd filtering" to my other projects?
5. The bandpass filter is here connected to SA612 by a simple transistor amplifier. Is this really improving the signal? I wonder if adding the same simple tansistor amplifier to my MC3361 receiver would help?
6. How would I add a fine tune? I consider just adding a second potentiometer, or maybe adding a second pot+second capacitance diode would be better...
Thanks in advance!
1-If GBWPs are different, you will not able to get a sufficient signal amplitude, so a proper amplifier ( higher GBWP ) should be used. It also depends on IF frequency, it may work (?)
2-In order to change the receiving frequency, you should tweak the circuit especially LNA at the input.SA612s oscillator circuit should also be modified ( for same IF frequency)
4-100uH is RFC, not for stabilisation.
5-I's not a simple amplifier even tough the frequency is low. It's just an matched LNA. I think you can use the same amplifier but I don't know the specs. of MC3361.It should normally work..
6-You can change C24 as a trimmer capacitor while decreasing C20 as-for instance- 33pF ( more or less) to decease the tuning sensitivity of the tank circuit by varicap. There are absolutely also some other techniques but this may help you to fine tune the tank circuit.
3-SA612 is still available, why you would use NE602 ?
1. in this application the GBWP isn't of importance, they only carry low audio frequencies. The wider GBWP of the MCP601 will do no harm.
2. should be OK, you need to reduce the values of L5 and C24 so they resonate at 27MHz and reduce the values of L2 and L3 to center the input filter on 27MHz.
3. Yes, for practical purposes they are the same device.
4. It's there to stop noise and signal being carried through the supply lines. It won't change the receiver sensitivity but it will reduce the background noise level and help to keep the circuit stable. It's usually a good idea to filter the supplies to sensitive areas of any circuit but how effective it is would depend on how much interference and it's frequency content.
5. As shown the amplifier wont work anyway, it needs a coupling capacitor before reaching the transistor and R13. It will give some amplification which is good but there are far better ways of making an RF amplifier than that.
6. duplicate D1/C20 but make the capacitor much smaller, I suggest 5.6pF. Then feed the 'fine' tune voltage in the same way as the main tuning voltage.
Brian.