VFO: Several  Dipper Ocillators

12/23  2012

after the "end of the world", i want a dipper. there is many choice to do this.  to be a dip oscillator, several feature to the oscillator is very important:

• wide range oscillation,  few hundreds khz to fen hundreds Mhz. 

• keep amplitude stable as possible, especially in one coil range.

• no strong ALC/no too strong oscillation to prevent  dip.

a choice is the VCO: Ultra Negative 100kHz-100Mhz.  i had build a amplitude stabilized VCO from  VCO: Ultra Negative 100kHz-100Mhz's secondary oscillator, that's low greenery tank  and bad phase noise, low power out.  this a upgraded version of high energy tank negative VFO with little modified.

1) C7 is disconnect, with C7, very many band can not start oscillation[ C7 must connected@ 2013/1/8]

2) add T3 as a buffer

3) add T4 and detector network for stabilized the amplitude

4) C1 is 365pF AM variable air capacitor, select corresponding Air Coil for each band.

500khz to 30Mhz output level around 1.5Vpeak. when >45Mhz output level dropped but still around 200mVp to 600mVp. better than Sweep: HF signal source [on going]]  which also a derive version of VCO: Ultra Negative 100kHz-100Mhz, only given 200mVp output and with noticeable amplitude fluctuations.

this version enhance ALC:

1) use in60 as detector

2) use T5 increase the control range.

result:

amplitude stable now, but dip can not be noticed, cause ALC eliminate it.  and the ALC is not good enough, the waveform is very bad at low frequency.

SM0VPO introduce a good design for dip oscillator, refer to http://www.sm0vpo.com/use/gdo.htm

this configuration have several features make sure very wide range oscillation is possible.

1. Vackar like configuration, variable capacitance in both arm of tank. so feed back is vary when tune, make it easy start oscillation and keep amplitude stable.

2. Coil (L1) taped, for every different coil, the feed back is adjust for best perfermance. 

3. tapped coil is inconvenient, you have to use five DIN socket for built the dip meter.

by remove the L1 tap, make it easy build with RCA sock or BNC. 

but when you tune the variable capacitor to max capacitance, the oscillation will stop. so add RFC to address this problem:

RFC1 ,RFC2  guard a different frequency range.

R3: for low frequency

RFC2: for HF and low VHF frequency

RFC1: for 29Mhz to 250Mhz

replace Q1 with MPF102, can work at least 400khz to 150Mhz.

2n5485 will work up to 250Mhz and more, layout must carefully designed.

cause of different JFET had different Gm, it can not cover a whole coil's oscillation range due to lack of feed back. here give a

enhancement version make sure easy start oscillation.

1) enlarge gain:  reduce R2 too 100R.

2)  parallel a C4 to hot end of the tank, this make most of energy feed back to Q1's Gate.

with these 2 modification:

*easy start oscillation in any position of the Variable capacitor

*cover 30Mhz to 120Mhz in one coil

*bad coil is also suitable.

1. 1Mhz to 240Mhz

2. low inductance coil need reduce R5 to 470R or less. otherwise when C8,C9 reach max value, oscillation stop

ECL negative: DC coupled 

@2013/1/8

i built this sure fire oscillator. from 200khz to 150Mhz even more, it's sure fire cause all DC coupled. it's good to use be a DIP, tune the P1 for a perfect sine waveform(for dip). for any coil, even very low Q inductor, it's still fire oscillation. For a dip, some coil might need parrlel a resistor to low Q for a noticeable dip.

above circuit seem like a multivariate oscillator, we transform it to another schematic style, now it's more clear,  it's ever introduced by <EMRFD>.

testing result:

1) frequency range 500khz to 120Mhz ,

2) Easy Oscillation 

2) when measuring  the frequency,  the counter tip will affect the oscillator,  need a buffer.