Saturday, November 21, 2015

Simpleceiver ~ Part 14

More of the Bits & Pieces

This will be a short posting to provide more information and give rise to other possible configurations. Previously we mentioned about the hard core homebrewer's who did not consider this a homebrew project unless it was totally homebrew. To that end, today, I hooked a 5 MHz LC VFO I had that was left over from a prior project wherein it was replaced with a Si5351. In further response to the hard core group, in the future I will present a design for a crystal switched VXO so that segments of the 40 Meter band can be covered using the VXO. But for now here is the LC VFO video. Note how quiet the receiver is between stations and also note I prototyped a 4 pole crystal filter that is "haywired" into the circuit.
In the most recent SolderSmoke Podcast #182 (11/20/2015) I mentioned to my friend and host Bill N2CQR about the power of the LT Spice simulations that were being used with this project and now I would like to expand a bit more on what was said.
In retrospect I am guilty of just trying things and then at times are a bit surprised when an expected performance level is on the short side. Below is the 12 MHz IF amplifier circuits that will be used in conjunction with the new 4 pole Crystal Filter. The current configuration is set to use 12 Volts to power the circuit. But I wanted to explore what happened at lower voltage levels. So OK I was toying with a spin off project called the Cellceiver. I have a big box of defunct mobile phones and in that box are a large number of still good rechargeable batteries. So the idea was to build a Simpleceiver powered from 3.8 Volts. Also important is the situation where you don't have +12 VDC and you grab a 9 VDC transistor radio battery thinking "this will work OK". Well maybe not. [Note in the schematic below the top margin got cut off but the top end of 68K gets connected to the +12VDC Rail.]
Now what happens if you change the Supply voltage [Going from +12 VDC down to + 5 VDC]?
This is amazing! As you drop the supply voltage from 12 VDC to 5 VDC, the gain drops by 2/3 from 18 dB to about 6.5 dB which is a dramatic reduction. Dropping to 9 VDC drops the gain by 4 dB thus if the circuit calls for 12 VDC then that is what it needs!
Take a look at the tank circuit and note that there is a capacitive voltage divider, C1 and C2. The equivalent series capacitance across the tank is about 23.5 PF. If you were to connect a 23.5 PF cap in series with a 10 NF that essentially is a 23.5 Capacitor. Run the simulation and look at the output level. Try putting the 23.5 PF cap on top and the 10 NF on the bottom and run the simulation. Then reverse the two with the 23.5 PF cap on top and the 10 NF on the bottom. Note the gain readings. You can draw some very important conclusions from this --which I will leave to the reader. Hint: Size and order matters!
Pete N6QW


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