EMRFD Message Archive 10824

Message Date From Subject
10824 2015-03-13 12:31:45 Nick Kennedy Audio level for DSB full modulation with DBM modulator
Is there a specific audio level to give 100% modulation when using a double balanced mixer to generate DSB? Say I'm driving the L.O. port (or should I call it RF?) with +7 dBm.  

Also, should the AF port be driven from a 50 ohm source just like the L.O. port?

I was reading about generation of DSB with diode mixers in SSDRA.  One caution was to not use off the shelf DBMs as they don't give a means to null the carrier.  Still as true today as in 1977?

73-

Nick, WA5BDU
10831 2015-03-14 02:27:04 Bill Meara Re: Audio level for DSB full modulation with DBM modulator
Nick:   The way I look at it, you have to make sure that, if you have the RF signal turning the diodes on and off, the audio signal should not be strong enough to ALSO be turning those diodes on and off.  

As for the off the shelf DBMs, EMRFD has a great mod for those SBL-1.  It is from Pete Juliano, N6QW.   He describes how to connect a small pot to the device to allow you to balance out any remaining carrier. 

73  Bill N2CQR 
10843 2015-03-14 23:22:04 vasilyivanenko Re: Audio level for DSB full modulation with DBM modulator
Big Thanks Bill

to clarify - adding a balance pot to a DRM etc.
 wasfirst copyright in 1963 by none other than Donald L. Stoner,  W6TNS

It also was featured in SSB works by EE  L.A Earnshaw in the 1960s and in Russian language literature published in the 1970s.


Thanks and best regards!
Todd
10844 2015-03-15 06:24:44 kb1gmx Re: Audio level for DSB full modulation with DBM modulator
It will be about 10db less than LO drive.

Note keep all ports at 50 ohms.  you can overdrive a level-7 (7dbm) DBM to as far as
13dbm no problem and that buys some headroom.

The easiest way to know is drive it with a clean sine on the audio and watch the RF 
out, watch for the DSB waveform to start compressing then flat-topping.  See that back up 3db.

For years, ok decades, I've used a 4 diode ring modulator with carrier balance pot.
I think its still in the handbook and has been since dirt was new.


Allison
10845 2015-03-15 07:14:55 Nick Kennedy Re: Audio level for DSB full modulation with DBM modulator
I was wondering how Pet got the lid off of that SBL-1.  I pulled one out of the parts bin and it looks like it's closed up pretty tight.

I also saw that EMRFD noted that current types of manufactured units might give good suppression without adjustment.

73-

Nick, WA5BDU

10846 2015-03-15 10:09:07 kb1gmx Re: Audio level for DSB full modulation with DBM modulator
The older SBL1s are soldered, newer are welded.  Dead ones are can
opened(desctructive).  I've gone around the edge of the welded ones 
with a diamond cutoff blade for forensic eval on failed ones.

To add the pot you just take advantage of the fact that all the ends of the
trifilar coils come to leads on SBL-1.  Easy hookup.

Also the difference is the typical of most DBMs is 35-45DB carrier suppression
(LO to RF isolation).  With the pot it can easily exceed 50DB worst case.

Suggestion, if you want good carrier suppression and higher output
build your own DBM using 1n914/4148 and add the pot.  You need a 
bit more LO but the result is a bit higher output (12dbm LO nets about 
0-3dbm RF easily)as well.


Allison
10847 2015-03-15 10:18:26 Ashhar Farhan Re: Audio level for DSB full modulation with DBM modulator
the answer is 42. but, as douglas adams asked, 'what is the question?'

if you are shooting for distortion free audio, then it should be very low. each db of drop in the audio level will lead to 3db drop in the distortion. a diode mixer is capable of 15dbm IIP3. that would mean, that at -10dbm audio injection, we are looking at distortion being more than 60 db down. very good i must say. you could go even lower, but you would be uncomfortably close to the carrier levels.

lower audio injection means better sounding audio but lower sideband-to-carrier ratio (or bad carrier suppression), -10dbm, that is 100mV seems like a nice compromise. but really, i am just quoting rick from EMRFD.

- f

10848 2015-03-15 12:27:16 Nick Kennedy Re: Audio level for DSB full modulation with DBM modulator
Thanks for the good info, Allison.

Now I see a couple circuits in EMRFD where at TUF-1 is used without any additional means of nulling the carrier.  One is figure 6.152.  Is shows RF input at 7 dBm and audio of 0.4 Vp-p, which is -4 dBm, so in the range you mentioned.

Yet another example is in figure 6.132.  Here a TUF-3 is used as a modulator, with +7 dBm of RF and 2.5 Vpp of audio.

The discussion of adding the null pot to the SBL-1 is in figure 6-101.  It does credit Pete, W6JFR.  Maybe that's for this specific mixer and not generically.  I misunderstood what it takes to insert the pot -- it's not necessary to "go inside" the package.

73-

Nick, WA5BDU

10849 2015-03-15 14:08:34 kb1gmx Re: Audio level for DSB full modulation with DBM modulator
>Yet another example is in figure 6.132.  Here a TUF-3 is used as a modulator, with +7 dBm of RF and 2.5 Vpp of audio.

Look carefully.  Thats 2.5VPP appled to a series resistor of 1K then 51ohm parallel load.
That results in .121V PP appled to the modulator.

You can push a little harder with acceptable distortion but not more than 3DB (-7dbm 
audio with 7dbm drive).

Yes the SBL-1 brings out the needed pins to make adding balance pot easy. Nulling the 
carrier helps with carrier to signal as will upping the drive to 10dbm. 

Distortion more than 35db down is adequate for filter rigs as the filter will 
take out the out of band stuff and the IMD of the power amps is in that range.
Keep in mind is the best way to increase "talk power" or average power is to 
take the filtered IF signal, clip it and then filter it again.  It keeps the distortion 
in-band and doesn't sound as nice but the signal has high average 
power without splatter.


For image reject rigs (IQ analog or SDR) a good carrier to signal without
distortion is best described as mandatory. As distortion will creep into the 
opposing sideband as buckshot/splatter ad degrade the signal overall.


Allison
10850 2015-03-15 16:03:27 Mike Re: Audio level for DSB full modulation with DBM modulator
Regarding homebrew diode DBM's:  There's a very interesting (to me, at least) webpage at where Nicole Fieraru discusses "Comparative Measurements on Double Balanced Mixers", based on "built-up" homebrew diode mixers, of various different diode types (1n4148, 1n60, 1n5711, etc).  Nicole has provided some comprehensive measurement data.  As near as I can tell, the different mixers built by Nicole all use a pair of toroid (or binocular) cores, along with 4 diodes, but without balance pot(s).

Mike Dinolfo N4MWP

10852 2015-03-15 20:41:48 vasilyivanenko Re: Audio level for DSB full modulation with DBM modulator
This thread really shines:

In EMRFD, Wes wrote ..."as used by W6JFR" .. Pete was 1 of the clever SSB homebrewers who applied a balance pot to boost carrier suppression in their balanced modulators.

By the mid 70's, keen Canadian Hams (who could afford them) figured out that the MCL SRA-1 package made for an easy mod with a balancing pot. The SRA-1 features the same pin-out and is a cousin to the SBL-1.
I'm sure this happened worldwide.

Finally " ...Alison wrote Keep in mind is the best way to increase "talk power" or average power is to take the filtered IF signal, clip it and then filter it again"  --- Do you have any circuit examples or schematics to share please?

Thanks and cordials,
Todd -VE7BPO
10853 2015-03-15 22:16:41 John Marshall Re: Audio level for DSB full modulation with DBM modulator
Todd, try W7ZOI's "A QRP SSB/CW Transceiver for 14 MHz" in December 1989 and January 1990 QST. The transmit chain is as Allison described - balanced modulator, IF filter, diode clipper, IF filter.

John, KU4AF
Pittsboro, NC

10859 2015-03-16 13:36:26 vasilyivanenko Re: Audio level for DSB full modulation with DBM modulator
Thanks John -- Wes emailed me and told me the same!
73, Todd