–8–

AN-502

total thermal noise into the ADC. Thus overall the analog

portion of the system and not the ADC determines SNR

for this receiver. Based on this data, a minimum SNR of

10 dB exists with an input at the reference sensitivity.

This is enough to satisfy the requirements of equaliza-

tion. Better sensitivity can be achieved by selection of

quieter analog components, especially the band select

filter and the low noise amplifier.

SAW Filter Requirements Continued

As mentioned earlier, the SAW filter provides some

amount of receiver selectivity. However the primary

goal of the SAW filter is to prevent the adjacent signals

from desensitizing the RSSI of the AD6600.

With a signal 3 dB above the reference level of

–104 dBm, the ADC input level is –63.3 dBm. At this

point, the SNR is 13.9 dB. The key is to maintain this

SNR as various blockers and interferers come in-band.

This is the purpose of the SAW filters. Since the AD6600

is a gain ranging ADC, out of band signals of sufficient

amplitude can desensitize the AD6600 to the desired sig-

nals. For example, if a signal passes to the ADC with a

level above –32 dBm, the first RSSI trip point will be

reached and cause the front end to attenuate the input

by 6 dB. This would reduce the SNR by 6 dB. Thus, any

undesired signals must be kept at such a level that the

input of the AD6600 is kept below –32 dBm on the high

side or below –38 dBm on the low side. This is because

the AD6600 employs gain hystereses to prevent gain

‘chattering’ during normal signal fluctuations.

Nor-

mally, the upper trip point of –32 dBm can be used since

normal power up ramping of the transmitted signal will

cause the RSSI hystereses control to reset between us-

ers. The lower trip point is used in the case of fading

profiles when the signal magnitude is falling.

Table I.

Interferer and

Blocker

Level

SAW

AD6620

200 kHz

+9 dBc

0

18

400 kHz

+41 dBc

25 dB

25 dB

600 kHz

–26 dBm

37 dB

45 dB

800 kHz

–18 dBm

47 dB

45 dB

>3 MHz

–13 dBm

104 dB

Table I shows the interferers and blockers of interest.

Absent from this list is the 600 kHz interferer (Section

6.3) because it is equivalent to a blocker at –35 dBm,

9 dB below the interferer, which should not disrupt

either the AD6600 performance or the equalization. The

600 kHz blocking specification is included in the Table I.

For the interferer specs, the desired signal is 20 dB

above the reference point. This generates an SNR of

30.9 dB on an input signal level of –46.3.

With a 200 kHz interferer, the interferer is 9 dB above the

signal level at the input, and the analog filters internal to

the AD6600 provide little or no rejection. Therefore, at

the ADC input, the interferer is about –35 dB. This is not

enough to trip the RSSI unless peaking occurs. If this

does occur only 1 RSSI step will be inserted causing the

SNR to fall 6 dB to 24.9 dB. This is more than enough for

equalization with an acceptably low FER.

In the case of the 400 kHz interferer, the signal level on

the radio input is 41 dB above the desired signal. Since

each of the two SAW filters provide 13 dB of rejection, a

total of 26 dB can be expected for a net signal level 15 dB

above the desired signal. Thus with the desired signal

of –46.3 dBm, the interferer is –31.3 dBm which would

cause 1 RSSI step to be inserted and possibly two. In the

later case 12 dB of SNR loss would result for a net SNR

of 19 dB, still more than adequate for a low frame error

rate (FER).

In the case of the blocker tests, the desired signal is 3 dB

above the reference sensitivity of –101 dBm. As before,

this gives an SNR of 13.9 dB. Now in the 600 kHz case, a

maximum block level of –26 dBm must be tolerated. In

this case, a minimum rejection of 23 dB is needed from

each SAW filter for a total of 46 dB.

Thus the input

blocker is (–26 + 37.7 – 46) or –34.3 dBm, which is below

the RSSI threshold.

Between 800 kHz and 3 MHz, the receiver must tolerate

blockers at –16 dBm. From the filter information, the

SAW filters provide a total rejection of 56 dB. As before,

the blocker is (–16 + 37.7 – 56) or –34.3 dBm, again below

the RSSI threshold.

Finally, blockers outside 3 MHz may be up to –13 dBm.

The SAW filters provide 106 dB of rejection to these sig-

nals. Thus, ADC input levels to >3 MHz blockers are

(–13 + 37.7 – 106) or –82 dBm, which are well below any

RSSI thresholds. Furthermore, the desired signal is at

(–101+37.7) or –63.3 dBm.

Since the sample rate is

6.5 MHz and our signal placement puts the signal at 1/4

the clock rate (about 1.6 MHz), the potential exists for the

blocker to alias directly on top of our desired signal. In

this case however, we have a C/I ratio of (–63.3 – –82.0)

or 18.7 dB, 9 dB better than the equalizer is required to

tolerate from a co-channel interferer!

Looking at the SAW filter plots below it can be seen that

actual rejections are better than those shown by these

calculations. This provides greater insurance that RSSI

gain stages will not switch improperly and reduce the

SNR below 13 dB.

To achieve 106 dB of out of band rejection, two filters

must be cascaded. One filter could be used as a roofing

filter; however, the burden of the channeling filter would

then be placed on a single filter. Therefore, two identical

filters would allow the performance to be split equally