I don't mean to hijack the thread but:
The 146 is just a bit before my time - what is the main difference between a 148 or 142? I see it doesn't have a SWR meter built in but is the board different?
(
... a bit before your time?...)
(While everyone in my younger days of schools of thought read Playboy and Hustler, I read (and drew) schematics and read works from a guy names Howard Sams)
Ouch...
And You're Welcome...
This could go long...so you know...
But the 148/142's One being Mobile the latter being a Base both used a different set of rules than the 146...
The 146 was closer Cousin to the PC-122 and AX/AR-144 lines...
- Used a similar PLL process, the 2824 PLL which was similar to the MB8719/34, but made thru a different manufacturer so they processed a little differently (Copyrights, Trade Marks - E&C) but to help streamline the approval process (Read Type Accepted FCC) they performed similar "external" functions so they operated similar.
- Both types use a "Loop" to reference or sample an external frequency "Tap" and compare it to the internal frequency it uses for itself as a means to Beat in Sync - using a slower frequency but the higher frequency was "divided" down (Heterodyned sample then placed thru a counter/divider) then sampled for error correction (that result), then the PLL either kicked the oscillator in that external loop higher or lower in speed (Varactor) to meet the expected frequency as a means to "Zero beat"
- Grants, 148'use a different set of frequencies that go above the typical CB band and above 10-meter into the Low-VHF business band (+35MHz) and used 7.8MHz to Subtract from that 35MHz to obtain 27MHz CB band (Generalized)
- The 146 is a SINGLE conversion, while Grant and Cobra line, the MB8719 used Dual conversion. - an extra step to obtain the SSB signal (~10.6 / .7 MHz IF and secondary IF of 16.4 MHz mixed to obtain 27MHz)
- Key things to think about...
- Audio IF signal for 146 used;
- - 10.695 MHz for AM
- - 10.6925 MHz for LSB
- - 10.6975 MHz for USB
- See the differences? approximately 2.5kHz (2,500Hz) and even narrower for Band pass audio bandwidth to reduce leakage into those other two frequencies
- So the PC-122 / 146GTL worked Upwards (Up-Mix), while the 148/Grant systems worked Downwards (Down Mix) when it came to the Transmitter stage and how it obtained the frequencies you use on the CB.
- You also have that extra cushion in the filtering to reduce the error in multiplication or, the mixing products for the error percentage on a lower frequency, that error percentage is AMPLIFIED to a higher level of error-rate in higher frequencies you mix up to as a factor of multiplication factor to get a newer frequency
- So in one way, the Grant and 148 / 142 provided a greater degree of accuracy for 35MHz - for 27MHz factoring. However that did not always equate to better control in the subtraction (or De-conversion) for the main culprit was the 7.8MHz signal being the added into the mess that was not "clocked" nor error corrected by the Radios' PLL system - only it's own VCO. The local IF was used for Decode and Encode that did not even look at the 7.8MHz on-board Xtal - only mixed with - to bring it up or down
The conversion process for both radios has their drawbacks...
The 146 being single conversion didn't have the luxury of extra filter steps in processing to remove RF captured noises (RF interference) like the Grant and 148's have available to them.
The need for extra sets of frequencies in which to accomplish some of the simpler tasks we don't even think about - required several layers of support not just in frequencies and their parts, but the effort of all those extra steps required for two layers of de-conversion for just a little bit better sound of SSB or AM signal processing
- - make it difficult to place it in a smaller wrapper (case)
- - so to keep parts count low and to use a more basic platform they used simpler processing pathway.
- - They also did away with many Bells and Whistles to save Real Estate and # of Switches to play with...
- - the 146 GTL used (borrowed?) a trick from the PC-122 line and gave us the Single Conversion process and used a filter system more commonly used today and did away with the 7.8MHz Local IF for 455kHz and used this single conversion step (27MHz, to 16MHz IF converted to 10.695 MHz) to obtain audio directly zero beating that against the SSB RX signal
- - SSB like FM, the Audio is the SHIFTED frequency you hear - the Radios' on-board IF remains steady to beat against it.
You did ask, hope this helps, I'm sure someone whom may be more knowledgeable will come by and dissect my post and point out errors omission obituaries and oblituaries (Pun) - in the effort of adding more to the conversation.
Stay tuned...
