Vertex Standard FT-60R Dokumentacja Pobierz pdf (Strona 5)

From February 2005 QST © ARRL

M. Tracy, “

QST

Product Reviews—In Depth,

In English,”

QST

, Aug 2004, pp 32-36.

that’s the rub. Many receivers are de-

signed to do everything—CW, SSB, AM

and FM and thus settle on a roofing filter

that is 15 to 20 kHz wide. The problem

is that if you’re a contester you end up

with all those signals ±7.5 to 10 kHz

away. In addition, the filter skirts may not

be steep, so you may get signals even fur-

ther removed. Note that IMD from sig-

nals spaced at 20 and 40 kHz (the 20 kHz

IMD data in QST Product Reviews

)

comes out looking good with this filter,

but not so for signals at 5 and 10 kHz

(the recently added 5 kHz IMD reports).

The best of the newest transceivers

solve this problem by having selectable

roofing filters to allow an optimum choice

for each mode. The Ten-Tec Orion is no-

table in having optional roofing filters as

tight as 0.25 kHz bandwidth. The ICOM

IC-7800 has two (15 kHz and 6 kHz) and

the soon to be released Yaesu FTDX-

9000 promises three (15, 6 and 3 kHz).

Single conversion receivers, such as the

Elecraft K2, can set both the roofing and

selectivity with a single filter following

the mixer.

The Ten-Tec Orion (main receiver)

and Elecraft K2 are ham band only de-

signs and thus have their first IF and roof-

ing filters in the HF region. The ICOM

and Yaesu transceivers include general

coverage receivers with an upconverting

architecture to a first IF in the VHF re-

gion. Narrow bandwidth steep skirted

crystal filters in the HF region are much

easier to make than those for the VHF

region and only recently have filters as

tight as those by International Radio been

available.

Note that the roofing filters are gener-

ally wider than the selectivity setting fil-

ter (except for single conversion designs

such as the K2). Thus, neither of the in-

Table 2

Comparison of Two FT-1000MP Series Transceivers With

and Without International Radio Roofing Filter Modification

Yaesu Mark V FT-1000MP, serial number 0K120017

Data taken on 20 meters with pre-amp off

Parameter

Unmodified

Modified

Noise Floor –123 dBm –130 dBm

Audio/IF Bandwidth 600 Hz 470 Hz

Blocking Dynamic Range, 20 kHz 130 dB 146 dB*

5 kHz 105 dB 130 dB*

2 kHz 102 dB 105 dB

1 kHz 104 dB

106 dB

IMD Dynamic Range, 20 kHz 88 dB 93 dB

5 kHz 76 dB 89 dB*

2 kHz 69 dB 79 dB*

1 kHz 68 dB 69 dB*

*Measurement was noise limited at the value indicated.

Some filter blow-by was noted on this measurement.

Yaesu FT-1000MP, serial number 7J21036

Data taken on 20 meters with pre-amp off

Parameter

Unmodified

Modified

Noise Floor –123 dBm –126 dBm

Audio/IF Bandwidth 600 Hz 470 Hz

Blocking Dynamic Range, 20 kHz 138 dB 141 dB

5 kHz 111 dB 128 dB

2 kHz 106 dB 108 dB

1 kHz 102 dB* 103 dB

IMD Dynamic Range, 20 kHz 94 dB 100 dB

5 kHz 76 dB 90 dB*

2 kHz 69 dB 71 dB

1 kHz 68 dB* 68 dB*

*Measurement was noise limited at the value indicated.

Figure 4—The roofing filter installed in a Mark V FT-1000MP.

Figure 3—The International Radio roofing filter.

surement point (5 and 10 kHz, for the

5 kHz spacing, for example), in real life

we get a spurious signal from every such

pair. Thus we will get a 3OIMD response

from 5 and 10 kHz off frequency signals,

but also from 3.3 and 6.6 kHz and –4 and

–8 kHz and 2 and 4 and 8 kHz, etc. This is

why we usually don’t just hear 3OIMD as

a single signal—it just sounds like junk.

So What Can We

About

3OIMD?

The key to fighting 3OIMD is to keep

the interfering signals out of the stage that

is generating the problem response. The

optimum way to do this is to filter right

at the antenna. This can be effective, but

is really only feasible if you only oper-

ate on a single or small number of fre-

quencies. The most frequently used

approach these days is to design an RF

stage (if needed at all) and first mixer that

can handle the large signals with mini-

mum distortion, and follow immediately

with a roofing filter. The roofing filter

need only be as wide as the widest band-

width of signals you wish to process

further downstream in the receiver—but

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