Sunday, 22 April 2012

Multiband VFO ver 1.2

New Multiband VFO

Some changes on the VFO
1. Replaced the SA612 with a hombrewed diode ring mixer
2. Replaced the Dual tuned 1.0 Mhz bandpass filters with a Triple tuned 2.0 Mhz band pass filters
3. Diode switched crystal oscillators to change band.
4. Change from 2 stage to 3 Stage RF amplification to provide 13 to 19dbm output.

Some notes
The construction on a single Copper clad board instead of multiple modules. a purely manhattan construction was change to a mix of manhattan and ugly construction.  The original plan was for the crystals to provide 7dbm to 9dbm output this was adjusted to 10 to 13dbm. The intention was to allow adding a 3db attenuator and still have enough power to drive the mixer.

Frequency Coverage Plan

VFO HFO IF Coverage Band Target
3.500 4.005 11 0 7.4997 6.995 40m 7.300 7.000
3.500 4.005 18 0 14.4997 13.995 20m 14.350 14.000
3.500 4.005 25 0 21.4997 20.995 15m 21.450 21.000

Power levels between stages (roughly measured during construction)
1. VFO = -8dbm this is further reduced by a 3db pad.
2. Crystal oscillators/buffer = +9 to +10dbm originally a 3db pad was planned
3. Mixer output = -26dbm This is after a 6db pad.
4. First 2 stages RF amplifier = +6db to +7dbm
5. 3rd stage RF amplification = +13 to +19dbm - I did not have any other transistor available so I maybe loading the 2n3904 more than it can handle.

Diode switched crystal oscillators

VFO, Mixer and crystal oscillators. Some leveling
of +/- 1db was sacrificed to get 9 to 10dbm output

Nice textbook waveform from the new mixer.

Unofficial measurement of RF leak of -46dbm
Upper band has a lot more noise

The output of the new mixer is a lot nicer than the old SA612, but whether it is better I will not know until I do some measurements. Most of the measurements right now are quick check as I build the mixer.