K5AM - Receiver second-order IMD measurements

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Receiver second-order IMD measurements

The test signals are at 6 MHz and 8 MHz. The K5AM receiver is tuned
to 14 MHz; the table shows the second-order intercept point IP2.

To put the results in perspective, measurements for several factory-
built radios are also listed. A number of older radios are included; they
utilize a variety of elaborate and expensive RF tuning mechanisms;
it was of interest to see how effective they are with respect to second-
order IMD. The table includes production dates when available.
Measurements were made for the homebrew radio, the FT-1000MP,
and the boatanchors. Other data were taken from QST product reviews.

Receiver	IP2 (dBm)	Date
Yaesu FT-1000MP-Mark-V - VRF ON, see below for OFF	112	2000-
Collins 51S-1	98	1959-72
Yaesu FT-1000MP - ANT jack only, see below for RX jack	89	1995-2000
K5AM	83	1992
Drake 2-B	81	1961-65
Collins 51J-4	78	1952-62
Hammarlund HQ-129-X	77	1946-53
Collins 75A-4	74	1955-58
National HRO-5TA1	73	1944-47
Yaesu FT-1000MP - RX IN jack	72	1995-2000
Hammarlund SP-600	70	1950-72
Yaesu FT-1000MP-Mark-V - VRF OFF	69	2000-
Collins KWM-2	66	1959-75
Icom IC-756PRO	63
Kenwood TS-570S(G)	59
Ten-Tec Omni-VI-Plus	58
Icom IC-718	55
Yaesu FT-100	53
Ten-Tec Pegasus	44
Icom IC-706MKIIG	39
Yaesu FT-847	15

Comments. There are wide differences between receiver designs
which affect second-order IMD performance. For example, the Yaesu
FT-1000MP uses eleven 5th-degree apolar Chebychev band-pass
filters. A band-pass filter for the 12 to 15 MHz range is in use on the 20
meter band, where these measurements are made. In addition, the
Yaesu uses three high-pass filters to reject signals in the f/2 region;
these filters are included specifically in order to improve the second-
order dynamic range. The results are excellent. The table also shows
the results for the same radio using the RX IN jack. Unfortunately, this
configuration bypasses the high-pass filters, and the performance is
significantly degraded. Although measurements were not made on the
160 meter band, second-order IMD performance is also likely to be
degraded there; this may be a serious consideration if a beverage
antenna is used at the RX IN jack in areas with strong broadcast band
signals.
    The Yaesu FT-1000MP-Mark-V uses a selectable panel-tuned
preselector called VRF; the results are exceptional, although there is a
loss in sensitivity when the preselector is enabled.
    Another important design factor likely to affect second-order IMD
performance is the use of diode switching in the front end of the
factory-built radios. The high-pass filters in the Yaesu FT-1000MP are
relay-switched and placed ahead of the diodes that switch the band-
pass filters. No diodes are used in the homebrew radio.

Preselector Notes. Rejection of out-of-band signals has become one
of the most prominent measures of performance for modern
receivers. Most other performance factors, some even more
important, have reached levels in excess of practical requirements.
Modern factory-built radios have attacked the problem of out-of-
band signals with a variety of front-end filter arrangements, with
varying success.
    Older radios from the 1940s and earlier have one or more RF
amplifiers tuned along with the local oscillator by a multi-section air
variable capacitor; they often achieved front-end selectivity better
than many current factory-built radios. In the 1950s there appeared
radios with tunable IF strips, requiring separately-tuned RF
preselectors. The Collins 75A-4 uses slug-tuned circuits at both the
input and output of an RF amplifier. These are mechanically linked
to the tuning dial, resulting in continuous resonating of the two
front-end circuits to the signal frequency without operator
intervention.
    The Collins 51S-1 design is perhaps the ultimate example of this
older method; it has a double-tuned circuit ahead of the RF amplifier,
and another single-tuned circuit ahead of the first mixer. In effect,
there are 30 separate triple-tuned turret-selected RF circuits, for the
30 separate bands.
    The front-end circuits in the Drake 2-B are capacitor-tuned and
require peaking by the operator with a separate panel control. The
Collins KWM-2 also requires operator tuning of the receiver front-
end circuits, which are shared by the transmitter circuits, by means
of a separate panel control labeled "Exciter Tuning."
    The K5AM homebrew radio uses only a single operator-tuned high-Q
LC circuit for preselection; the second-order IMD performance is
adequate. High-pass filters could be easily added. Simple filters with
only 20 dB rejection of signals in the f/2 region would raise the second-
order intercept point by 40 dB.

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