RF AMPLIFIER and LOCAL OSCILLATOR TRACKING

[gvanproe]

I AM CONSTRUCTING AN AM BAND RECEIVER,USING A THREE GANG CAPACITOR. EACH CAPACITOR IS 570 PF WHEN CLOSED AND 11 PF WHEN FULLY OPENED. WHAT VALUE INDUCTORS DO I USE TO CAUSE THE RF STAGE TO TRACK THE LOCAL OSCILLATOR STAGE? THE IF FREQUENCY IS 455 KC. SO THE LOCAL OSCILLATOR FREQUENCY MUST BE 455 KC ABOVE THE RF FREQUENCY. PLATES CAN BE REMOVED FROM THE LOCAL OSCILLATOR SECTION OF THE VARIABLE CAPACITOR IF NEED BE.

[bodgy]

You will need padding capacitors on the oscillator side(assuming superhet operation) - I know you said AM but what exact freqency range do you need 560-1100Khz? and do you want the oscillator to be above or below the fundamental frequency and what I.F. frequency will you be using.<p>Basically without loadsa theory at this stage - you need the capacitance ratio for the RF and Amp stage -you have that - the frequency ratio required at the RF/Osc stage fro there the padding/tracking caps can be calculated.<p>bodgy<p>whoops sorry misread your post You've given the info - I'll start again next post<p>[ March 20, 2002: Message edited by: bodgy ]</p>

[bodgy]

I'll start again,<p>You will still need tracking/padding caps on the various stages.<p>The inductance required is going to be around 100uH at the RF stages.<p>The inductance formula is L=1/(6.28*f^2)*C and should be calculated at approx mid capacitance range or f=1/6.28*sqr L*C so using a 100uH coil the freq range is 1.759MHz to 0.666 MHz.<p>Your tuning ratio at RF is 2.64:1 and the Osc ratio is 1.97:1. Just add the IF frequency to your max/min freq range.<p>Your capacitance ratio needs to be squared so the cap ratio becomes 6.96 and 3.88 respectively<p>Now as you can see your RF ratio does not match your Osc ratio, and to track correctly, you need to fiddle with the Osc inductance and more easily add padder caps to the various stages.<p>To work through an example lets make it slightly easier on ourselves.<p>We'll take the tuning ratio that we want to be 3:1 so we're now looking at a cap ratio of 9:1.
So we take the Max capacitance of the tuning cap 570pF.<p>Min tuning capacitance will equal CmaxRF/Tuning ratio = 570pF/9 = CminRF = 63pF.<p>Trimmer Cap = CminRf-Cmin = 63pF-11pF = a trimmer cap that covers 52pF the trimmer cap is in parallel to the tuning cap<p>Padder capacitance is worked out by Cpad = Cmin * Cmax * ratio/Cmax-ratio * Cmin. Padders are in series with the tuning cap and will be used mainly in the Oscillator stage combined with a lower inductance coil.<p>Do the same math for the Oscillator stage.<p>Hopefully you're now totally confused and someone will point out I've been to engrossed in digital electronics and my RF theory has gone to pot.<p>So I'm off for a smoke to calm down - 'cept, I don't smoke - sigh<p>bodgy