Your simulation gave the same answer as I had posted in my previous note, which
is good because that helps solidify teach analysis. When two different analyses
give the same result, that’s pretty good evidence that you are on the right
track.
The higher bands cover more bandwidth so the filters would need to be different…
The loop IF in the Omni 5 and Omni 6, on the other hand, must provide pretty
constant output over a wide bandwidth. That PLL runs at 200 MHz and is divided
by 40 to get the 5.0 to 5.5 MHz output. In order to offer relatively flat
output for 500 MHz at 5 MHz, the IF loop filter needs to have a bandwidth of 20
MHz with no more than a few dB or so ripple in order to offer enough signal at
the PLL chip to lock.
That’s about 2.5 times the bandwidth of an old analog color TV video IF.
The feat is accomplished by only three elements, L1, L2 and L3.
Ten Tec was pretty good at designing broad band circuitry…
Gary
> On Feb 21, 2019, at 6:09 AM, Phil Erickson <phil.erickson@gmail.com> wrote:
>
> Hi again,
>
> I was hoping to avoid the words "coil rewind" but I guess you're right.
> Time to break out the magnet wire. Rats.
>
> By the way, for posterity, the bandpass filter topology is interesting
> and based on my crude ngSPICE simulation, it actually has two reasonably
> narrow resonant peaks, which are presumably located at the two LO
> frequencies needed. The simulation did not give these at a frequency I
> trust due to a degeneracy in the way they are actually implemented, but
> there are definitely two. The "front" coil L1 controls the low resonance,
> while the "back" coil L2 controls the high one. This only applies for the
> 160/80 and the 40/30 topology; the higher bands have 3 coils involved and
> are more complex.
>
> 73
> Phil
>
>
> On Wed, Feb 20, 2019 at 11:31 PM MadScientist <dukeshifi@comcast.net> wrote:
>
>> It’s always difficult to understand what drives a particular tech
>> writer/draftsperson in deciding what to include and what to not include in
>> a manual.
>>
>> I suspect that a true “service manual”, if it exists, would have this
>> information.
>>
>> While Tec Tec is to be lauded for its high quality documentation on the
>> Orion, for instance, that otherwise excellent manual completely omits
>> anything like a signal level diagram or a boot up sequence that would aid
>> repairs.
>>
>> That said, you do get the values for 160 and 80 meters, 0.68 and 0.47 uH.
>>
>> Now the circuits are different from band to band so simple scaling would
>> be difficult, but remember, these are adjustable so you don’t need to be
>> dead-on when selecting coils to replace these.
>>
>> The design constraint for this filter is that each set needs to have
>> sufficient bandwidth to cover two bands, since the tuned circuits are
>> shared by two bands, about 3.5 MHz.
>>
>> In band ripple is not a factor since this filter only needs to pass two
>> frequencies, the lower band and the higher band. It does not need to pass
>> any frequencies in between.
>>
>> The frequencies are rational, ~16 to 33 MHz so winding a coil should not
>> be all that tough.
>>
>> Can you not count the turns on the broken ones and rewind?
>>
>> I wish I had a board for you but my LO boards went away a long time ago.
>>
>>> On Feb 20, 2019, at 10:52 AM, Phil Erickson <phil.erickson@gmail.com>
>> wrote:
>>>
>>> Hi again,
>>>
>>> [Forgot to mention: I jumpered across this filter (from D41 to D36) with
>>> a 0.01uF capacitor and the receiver came to life on 40 and 30 meters, so
>>> this latest problem is definitely in the bandpass filter section
>>> attenuating the LO too much.]
>>>
>>> 73
>>> Phil
>>>
>>>
>>> On Wed, Feb 20, 2019 at 11:43 AM Phil Erickson <phil.erickson@gmail.com>
>>> wrote:
>>>
>>>> Hi all,
>>>>
>>>> (I hope someone else someday can make use of this VI+ debugging
>>>> information..)
>>>>
>>>> I'm still troubleshooting my VI+, which you can follow by searching the
>>>> archive. In particular, I'm still figuring out why the XO/LO board has
>> low
>>>> output drive for the LO on some bands (160/80/40/30), subsequently
>> causing
>>>> the BPF mixer to never turn on.
>>>>
>>>> Right now, I'm focusing on the 40/30 meter bandpass filter section
>> after
>>>> the U1 mixer but before the amplifier (Q6,Q7,Q8) and output. There are
>> two
>>>> tunable coils, L1 and L2. Under the microscope, at least one of these
>> is
>>>> definitely broken at its base and I'm sure that results in a very
>> mistuned
>>>> filter, which is what is causing my attenuation problem. I need to
>> replace
>>>> it and possibly more - someone was obviously in here with a 'golden
>>>> screwdriver' as we found epoxy as well. The adjustable coils are really
>>>> bad mechanically and the core wobbles a LOT when you move it with your
>>>> finger - easy to break.
>>>>
>>>> But Ten-Tec's schematic has no values of the appropriate inductance
>>>> range :( Ugh.
>>>>
>>>> Does anyone (Gary? Peter?) have info on the designed range for L1 - L11
>>>> for the bandpass sections on that board? I can't find it anywhere on
>> the
>>>> schematic and the manual doesn't say either. It's bizarre that they
>> label
>>>> some things but leave this out (?).
>>>>
>>>> Cheers/73
>>>> Phil W1PJE
>>>>
>>>>
>>> _______________________________________________
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>>
>> _______________________________________________
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>>
>
>
> --
> ----
> Phil Erickson
> phil.erickson@gmail.com
> _______________________________________________
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> TenTec@contesting.com
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