> The K3 uses a DSP IF at 15 kHz. The roofing filter is the
> primary means of suppressing the 15 kHz image on transmit
> and receive. In wideband modes (ESSB, AM and FM), this is
> especially true.
Let's see ... with a 15 KHz DSP IF, the image would be 30 KHz
minus the modulating frequency (for AM) from the center of the
transmit ("roofing") filter. Based on the specifications of
Inrad's 8 and 9 MHz AM filters (8 pole) and their 7 MHz (6
pole) front end filters (15 and 20 KHz wide at -1 dB), the
FM filter's shape factor should be in the 1.6 to 1.8 range
and the -60 dB bandwidth between 20 and 25 KHz. Unless the
filter or circuit layout has a major problem, the response
should down at least 90 dB at +/- 15 KHz.
With AM (worst case), assuming the carrier is centered in the
filter and a highest modulating frequency of 4 KHz (K3 limit
for ESSB), the image sideband is 26 KHz from the center of the
(transmit) IF filter - well into the filter stopband.
With an ultimate rejection of "only" 90 dB 25 KHz away from
the center of the filter, the transmitted image from a 1500
Watt (peak) amplifier will be 1.5 microwatts peak for an ESSB
signal, 0.75 microwatts for the carrier of an AM signal or
0.375 microwatts for the sideband of an AM signal. 90 dB of
image rejection is more than 30 dB better than the 5th order
IMD from most amateur 1500 watt amplifiers and 40 dB better
than the K3's own 9th order IMD performance (ARRL measurements).
73,
... Joe, W4TV
> -----Original Message-----
> From: [EMAIL PROTECTED]
> [mailto:[EMAIL PROTECTED] On Behalf Of Lyle Johnson
> Sent: Saturday, September 20, 2008 2:15 PM
> To: 'Elecraft Reflector'
> Subject: Re: [Elecraft] K3 configuration question
>
>
> >> I had the same thought when ordering my K3, but I chose
> >> the FM filter instead of the AM filter. That will give
> >> me sufficient bandwidth for good quality AM RX as well
> >> as FM TX/RX.
> >
> > Absolutely. It is a shame that the K3 firmware currently
> > blocks ESSB or AM transmit when the FM filter is installed
> > in place of the AM filter. Fortunately, if you're not using
> > FM transmit, it is possible to lie to the firmware and tell
> > it FL1 is 6 KHz.
>
> While you can do this, I suggest you not do so.
>
> Here's why.
>
> The K3 uses a DSP IF at 15 kHz. The roofing filter is the
> primary means
> of suppressing the 15 kHz image on transmit and receive. In wideband
> modes (ESSB, AM and FM), this is especially true.
>
> The wider bandwidth of the 13 kHz FM filter results in poorer
> suppression of the image. In the FM sub-bands, it is suppressed well
> within the definition of good engineering practice. Further,
> FM is not
> considered to be a weak-signal mode.
>
> SSB is often a weak signal mode, and ESSB and AM operations are
> typically within the image offset of potential weak signal
> operations.
> Thus, if you transmit in other than an FM sub-band using the
> FM filter,
> you risk interfering with nearby weak signal operation.
>
> We are studying the use of the FM filter for such purposes,
> but we have
> not yet completed our characterization of the use of the FM
> filter for
> non-FM use. When we do, and if the image suppression is
> sufficient to
> allow you to be a "good neighbor" when running your K3 to
> drive a legal
> limit amplifier such that the transmit image is in a nearby
> weak-signal
> area, we'll directly support its use in the K3 firmware.
>
> Until then, please consider the potential interference you
> may cause if
> you use the FM filter in this manner.
>
> 73,
>
> Lyle KK7P
>
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