Sweep: Build  new topology signal source

project start@2014/8/03

 band flatness response of a VCO is very important for sweep signal source. zoom oscillator  suitable for such a application.  i plan to build a array of VCO cover 300 khz to 50 Mhz range, use diodo as RF switching.

band switching control both the diode switch and the power of oscillator, so RF switch attenuation is not critical.

Full oscillator:

for each band, need adjust the bias resistor: 10k to 100k, even < 10k or > 100k.    basic policy is get enough gain from the feedback capacitor (larger one), then adjust bias until get flat response in whole band.  typical flatness , amplitude change will < 10%,  and even better.

First problem is,  tank energy is too high, make varactor bias several detune by signal, output waveform is severely distortion.  you can not lower the tank emerge by with reduce bias, because low bias make amplitude response not flatness.  i made a wrong decision, limit the tank energy by 2 diode.

problem of this circuit is, the tank energy is too low and tank is very lossy, after finished all system. i found it need a buffer to driven the diode switch. and most Annoying thing is,  after change to battery power, oscillation stopped, even with heavy bias, it still can not start oscillate, seem power rail need some noise to kick it up.

Failure reason check:

too compact layout, there are some short circuit!

ok, we learn something from first oscillator, the right way to setup a tank tolerant the high tank energy is use back to back varactor pair  which you still can get eye candy waveform.

another thing learn is a oscillator always want a buffer.  so i pick up a wide recognized buffer introduce by ARRL:

but, unfortunately,  the output from this buffer not be flat as VCO, from 1Mhz to 4Mhz amplitude reduced significantly.  i run a spice simulation of this circuit, AC sweep show it  Gain start roll down from 1Mhz,  what a coincident!  most of this response is BJT's miller capacitance and very high input couple resistor, which form a low pass RC filter. 

by simulation and get ideas from MMIC, i got this buffer, which provide about up to 200Mhz flatness response, and > 1k input impedance@100Mhz. if use one transistor as buffer,  input impedance will significantly small, most because bjt's input capacitance.  

A simulation low input capacitance not mean this buffer will severely loading  the tank make it stop, what really happen is the input capacitor will became a part of the tank, impact the frequency, and stability.

first time i use a easy to tapped tank, and i try connect the 50ohm load directly to the tank tap( 4:1 impedance ratio), amazing thing is, it could deliver 4V peak to peak voltage to 50ohm load.

so i decide keep this capability, left the bias 1.6k. but this decision still wrong.  such a strong signal(with out loading), will make diode switch turn into a frequency doubler, not a switch.  and if you connect 50ohm load after RF switch,  the current flow through the diode switch will clipped by diode, cause the diode bias current is not that enough.

after whole system being integrated together, one problem raising.  driven a 50 ohm Loading, oscillator can do , but RF switch can not. in another way, arrange the buffer before the RF switch,  is meanless to the sweep.  cause the oscillator could driven the RF switch, buffer provide 50ohm driven is useless before RF switch. (buffer for isolation is different thing.) 

the wide band oscillator need a wideband buffer, normal buffer can not offer wide band flatness,  so arrange buffer after the RF switch is a good choice.  and isolation the oscillator should use tank tap provide light couple to oscillator, this will provide another benefit, because tap reduce signal level, then easily pass through RF switch without distortion.

in general, this construction is useless! 

1. no frequency count  output

2. bias of RF switch is bad, signal is too strong to pass through the diode

3. oscillator coupled to RF switch is not well controlled.

3. 50 ohm loading testing failure on 4-10Mhz VCO, other VCO is fine but pass after RF switch,  no one work properly.

4. layout is too compact which prevent to do final adjust.

Better design:

1. use tapped inductor extract signal from VCO 

2. extracted signal should less than 100 mVpp and even smaller, then it can pass through RF switch without over driven the diode switch, and easy to be buffered by use emitter follower.

3. VCO bias should take the load into account.

4. should not try get large signal from VCO, better way is use RF amplifier after the buffer to get strong signal

5. back to back varactor as tank variable capacitor, better strong signal response.

In one word, the experimental sweep signal source is failure, but found a  way to achieve better result.

Band plan consideration:

455khz oscillator suffer with no suitable tank coil to construction, too big or too lossy.  this band should drop wideband sweeping function,  use a 455khz IF can, remove original embedded capacitor change to a smaller capacitor, and use varactor to form a VCO, could achieve 350khz to 550 khz range.

heterodyne signal source for low band sweep:   300khz to 3M, better cover should be heterodyne signal source, i.e, use 10Mhz crystal oscillator, and a 10 to 15Mhz oscillator, after mix, get 300khz to 3-5Mhz signal, cause 10mhz is a octave end to 5Mhz, 3:1 for 3Mhz,  the signal is easy get  very clean by a simple low pass filter.