[TMC] SBE-3 & SBE-6 on AM
Duncan Brown
duncanancy at earthlink.net
Sat Mar 1 17:57:00 EST 2014
John,
Thanks for your very detailed analysis!!
I am more familiar with the "standard" plate-modulated form of AM and
this is the first time I have had to wade into a rig where the AM is
regenerated by carrier insertion. The more i worked on it, the more I
was coming to the conclusion that it is not as simple as just feeding
some carrier back in to regenerate the AM signal. When I saw in the
SBE-8,-9,-10 that they had added a phase shift network to make "true AM"
I was pretty sure that the SBE-3 & -6 was not really designed to do real
AM. Thanks for the confirmation that this is what is going on. After
30 yrs in radio communications equipment design engineering, it does not
surprise me at all that the SBE-3 & -6 was primarily designed for
SSB/ISB and that AM was just an extra.
I will try to build up the phase shift circuit and see what that does
and also check out the SSB filters. I have experience working with
group delay in filters of digital radios (though that was 15-20 years
ago), but don't know if I can get my hands on a network analyzer any
more. The 250KHz notch filter seems OK as I get good carrier suppression
and the unwanted sideband suppression seems to be down at least 50 db.
The SBE-6 has 7.5 Khz BW filters, just like the SBE-3 (in fact I have
not found any difference between this SBE-6 unit and the SBE-3 manual).
I don't expect broadcast quality AM from the SBT-1KV, but it would be
nice if it worked on AM. If I can't improve the AM, I'll just use it on
SSB. But I'll try the phase shift network and check out the filters
first before I give up.
Thanks for all your help!!
Duncan
K2OEQ
On 01-Mar-14 10:49, John Vendely wrote:
> Hi Duncan,
>
> AM was already becoming outdated when these exciters were produced,
> and TMC regarded themselves as sideband specialists and considered AM
> largely irrelevant. So there was no effort to provide high quality
> AM in the SBE and SBG series exciters. Although you can insert a
> carrier and two sidebands, the resulting DSB AM is poor due to the
> disturbed phase relationships between the two sidebands, and between
> the sidebands and the reinserted carrier. This causes severe
> distortion when received on an AM receiver with conventional envelope
> detection. As you observed, no amount of sideband and carrier level
> adjustment corrects the problem. I have several SBE and SBG series
> exciters, all of which use the same crystal filters, and they all
> exhibit this problem. The problem mainly stems from group delay
> variations between the two sideband filters. As you noticed, TMC
> provided a carrier phase adjust in the later versions of these
> exciters, and this does help considerably, but it's only a
> compromise. It does nothing to compensate for the filter problems,
> and the correct phase relationship can never be obtained between the
> carrier and both sidebands at all modulating frequencies.
>
> You shouldn't see that severe modulation asymmetry you describe,
> though, which indicates more serious problems. Make sure the sideband
> filters are in good shape. I've found these old crystal lattice
> filters (made for TMC by Bulova Electronics), have often aged
> considerably, and are well out of their original specs. They often
> suffer from severe passband ripple and poor opposite sideband
> rejection. I'd say 75% of the SBE/SBG series exciters I've worked on
> had filter response problems of varying severity. You may have a badly
> aged and mismatched pair of filters in your exciter. Fortunately,
> they can sometimes be repaired and realigned, provided there are no
> dead crystals. I unsoldered the filter cans, and built up a test
> fixture to allow repair/realignment on a network analyzer, and was
> able to get the majority back in spec. Most problems seem to be from
> aged crystals, and the occasional defective capacitor. If the
> crystals haven't aged too far, the filter may be realigned with the
> internal piston trimmer capacitors. Also make sure the carrier notch
> filter hasn't failed or aged off freq and encroached on one of the
> sidebands. You probably would have noticed poor carrier suppression
> on SSB if this were the case.
>
> Filter alignment is tricky, and requires a vector or scalar analyzer
> with synthesized swept LO to do it right. A vector analyzer is really
> the way to go, as you can observe group delay and amplitude response
> together, which makes alignment much more convenient and gives a
> better overall response. The object is to get the optimum combination
> of passband frequency response and group delay, with good unwanted
> sideband rejection. It should be possible to get passband ripple down
> to within about 1.5 dB, and unwanted sideband rejection of 55-60 dB at
> 500 cps to 1kc into the opposite sideband. Rejection may degrade
> slightly above 1 kc, but should never drop below 50 dB. I should
> point out that all my exciters have the wideband, 7.5 kc BW sideband
> filters, not the 3 kc filters. I've never repaired one of the 3 kc
> jobs. They will have much worse group delay at around 3 kc than the
> 7.5 kc types, of course. The 3 kc BW, 250 kc CF filters used in the
> SBE-3 are completely different from those in the SBG-2, BTW.
>
> If you're really serious about using this transmitter on AM, I would
> recommend installing a carrier phase adjust network similar to that
> used on the SBE-9 exciter. This will definitely improve AM, but even
> with in-spec sideband filters, it won't be perfect. I guess it depends
> on how good you want it, but the bottom line is that two independently
> generated sidebands with a reinserted carrier is just not a real good
> way to make AM. These were among the best sideband exciters of their
> time, but AM was an afterthought, at best. TMC did it right in their
> later solid state MMX series exciters, which have a separate DSB
> full-carrier amplitude modulator, separate from the ISB modulator.
> These produce near broadcast-quality AM.
>
> Regarding the hum problem, if it's not a power supply problem, it may
> be a corroded chassis ground return in the audio section, as Richard
> K. suggested. I've seen this before, as has Mark KI0PF. It may be
> possible to re-tighten ground lug screws with a nutdriver.
>
> Best of luck, and let us all know how things develop...
>
> 73,
>
> John K9WT
>
>
>
> On 2/28/2014 10:10 PM, Duncan Brown wrote:
>> The AWA Museum has a SBT-1KV transmitter with a SBE-6 (similar to the
>> SBE-3) exciter. I tried to set it up for AM for the AWA QSO Party a
>> few weeks ago and the AM signal looked very distorted. I pulled the
>> SBE-6 and brought it home to work on, but have not been able to find
>> the problem.
>>
>> Basically, in AM, I can get 100% positive modulation, but only about
>> 50% negative modulation. Played around with audio levels and carrier
>> insertion, changed tubes, used external oscillators and realigned
>> everything, but to no avail. I wonder if others have had the same
>> problem???
>>
>> The problem seems to be in the 250Khz mixer/IF area. The only
>> conclusion that I have come to is that perhaps there is something
>> wrong with the way the 250Khz is reinserted with the sidebands. The
>> SBE-3 manual never talks about 100% carrier AM (only 10-50%).
>>
>> In the later SBE-8,9,10, there is an added 90 degree phase shift
>> circuit (that is not in the SBE-3 or -6) between the 250KHz
>> oscillator and the carrier insertion point (V126-1). The SBE-8,9,10
>> manual says that this is to provide a "true AM signal". Implying
>> that previous designs did _not_ provide a "true AM signal" ??? (It
>> is well within the realm of possibilities that the SBE-3 was
>> originally designed mainly for SSB and that full AM (by turning up
>> the Carrier Insert control all the way) was just thrown in as an
>> "extra". Then later on (after the SBE-6 was in production) some
>> customer wanted to use the SBE-x for real AM and the TMC engineers
>> realized that they had to correct the phase of the carrier inset, so
>> it was put into the SBE-8,9,10 version.)
>>
>> Has anyone with a SBE-3 or -6 had this problem??
>>
>>
>> Another problem I found with the SBE-6 is a lot of 60 hz hum that
>> seems to be getting into the signal in the 250Khz section. It is
>> about -40 to -50dbc on the output, but audible. I spent a lot of
>> time trying to track down the hum source thinking it might be
>> connected to the distortion, but never found a smoking gun. Probably
>> due to a bad ground somewhere and the fact that they run the tube
>> heater returns through the chassis. I tightened up all the tube
>> socket screws (they provide all the ground points for each tube) but
>> didn't make much difference. Is this a common problem?? (I'm thinking
>> about running separate wires for the filament returns in this section.)
>>
>> Thanks for any help/comments,
>>
>>
>> Duncan Brown, K2OEQ
>>
>> Antique Wireless Association Museum Asst. Curator, Commercial Equipment
>> (also Chief TTY operator & repairman)
>> http://www.antiquewireless.org/
>>
>>
>>
>> ---
>> This email is free from viruses and malware because avast! Antivirus
>> protection is active.
>> http://www.avast.com
>> ______________________________________________________________
>> TMC mailing list
>> Home: http://mailman.qth.net/mailman/listinfo/tmc
>> Help: http://mailman.qth.net/mmfaq.htm
>> Post: mailto:TMC at mailman.qth.net
>>
>> This list hosted by: http://www.qsl.net
>> Please help support this email list: http://www.qsl.net/donate.html
>>
>>
>
>
> ______________________________________________________________
> TMC mailing list
> Home: http://mailman.qth.net/mailman/listinfo/tmc
> Help: http://mailman.qth.net/mmfaq.htm
> Post: mailto:TMC at mailman.qth.net
>
> This list hosted by: http://www.qsl.net
> Please help support this email list: http://www.qsl.net/donate.html
>
---
This email is free from viruses and malware because avast! Antivirus protection is active.
http://www.avast.com
More information about the TMC
mailing list